Since 1st edition's multiclassed characters basically function one or two levels behind a single-classed character -- at least, until they hit a performance cap, usually somewhere just below "name" level (most demihumans cap out at 4th to 9th level for anything but thieves, with elven magic-users potentially peaking a bit higher) -- what would happen if this were tried with 3rd edition multiclassed characters? It might look something like this:
(Note: There are significant problems with this implementation, which I'll discuss afterward. Keep your hats on.)
You pursue a blend of the skills from two different adventuring professions.
This feat must be taken at 1st level.
BenefitChoose two classes. You gain the ability to blend those two classes' features as if they were a single hybrid class, so you might be a cleric/fighter 6, or a rogue/wizard 3. You can gain levels in this hybrid class as if it is one class, but most of your features function as if your class level is one level lower than your actual level. This provides you with the following characteristics:
Base Attack Bonus: Your Base Attack Bonus from your hybrid class equals the Base Attack Bonus from whichever class is better, as if your class level were one level lower. For instance, if you are a fighter/wizard 3, you use the Base Attack Bonus of a level 2 fighter, a +2 BAB.
Saving Throws: Use the base Saving Throws that are best in each category, as if you were one class level lower. For instance, the aforementioned fighter/wizard 3 gains the Fortitude saving throws of a level 2 fighter, and the Will saving throws of a level 2 wizard. The Reflex save is the same for both classes and thus is the same for the hybrid class.
Proficiencies: You gain all of the proficiencies granted by both classes.
Class Skills and Skill Points: You gain the class skills listed for both classes. You gain skill points from whichever class grants the most favorable total, as if you were one level lower -- for instance, a rogue/wizard 3 would gain skill points as if a 2nd-level rogue (because rogues gain more skill points than wizards).
Feats: You gain feats based on your total character level. Thus, a rogue/wizard 3 gains a feat for reaching 3rd level.
Class Features: You gain the class features of both classes, as if you are one level lower in each. All level-dependent features function as if you're one level lower than your hybrid class level. For instance, the aforementioned rogue/wizard 3 gains the sneak attack bonus of a level 2 rogue, the 2nd-level rogue's evasion class feature, the wizard's Scribe Scrolls feat, and the spellcasting ability of a level 2 wizard. The character doesn't yet have 2nd-level wizard spells, nor does the character have the trap sense bonus normally gained by a 3rd-level rogue.
Hit Dice and Hit Points: Average the hit dice of your two classes, and use that as your hit die type. Roll this hit die as appropriate for your total character level. For example, a rogue/wizard 3 would gain 1d6 hit dice from rogue levels and 1d4 hit dice from wizard levels. Averaging these provides a hit die type of 1d5 (roll 1d10, 1-2 = 1 point, 3-4 = 2 points, etc.), and the character would have a total of 3d5 hit dice.
So what does that get us? Essentially this is a pseudo-copy of how multiclassing worked in 1st and 2nd edition, to some degree. You get the better saving throw bonuses and attack rates, all of the proficiencies, all of the weapon and armor skills, and some of the class features, but you function as if you're a level behind the rest of the party.
Where does this fall down? Well, in several places, mostly because of departures in how 3e and later versions of D&D function compared to 1/2e.
Too Many Class Features: In 1/2e, the fighter class had few "class features" to speak of. The fighter's claim to fame was the ability to learn to use any weapon, to wear heavy armor, and to take all the punishment that the monsters could dish out. In 3e and later versions of the game, the fighter starts gaining extra class features, primarily in the form of feats. (Pathfinder starts taking this into the direction of giving the fighter more combat-specific bonuses that make the fighter the undisputed king of the battlefield, which is a good direction.) Unfortunately this means that when you make a multiclass character in 3e, you're no longer taking a set of class features from one class and layering on the better combat survival abilities of the fighter. You're taking the fighter's better combat ability AND class features and adding that to another class's features. This means that a character who is, functionally, a fighter 2/wizard 2 may not quite hit the peak performance levels of a fighter 3 (your BAB is one point lower) or a wizard 3 (you don't have second-level spells), but the hybrid functionality may well push you over the top. This becomes more pronounced when you look at multiclassing into similar classes. A fighter/barbarian hybrid under this system would lag slightly in terms of BAB but would still have phenomenal hit points and would get bonus feats and rage. This disparity becomes really pronounced (in bad ways) at very high levels. For a group of 20th-level characters, a fighter/barbarian 20 hybrid would have ten bonus feats as well as all the barbarian's best damage reduction, trap sense, and rage benefits (except mighty rage). The loss of one point of BAB is a small price to pay for this. (1e/2e prevents this to some degree by having classes in categories, and you can't multiclass in two classes that share the same category, so no fighter/ranger or rogue/bard for your enterprising demi-humans.) How to deal with this? Well, the problem is that at at higher levels, having all of these combined class features is more valuable than the base attack bonus and saving throw bonuses, but at lower levels, getting the best BAB and saving throws means a lot. When you have a choice of BAB +0 or +1, that +1 BAB is a big difference, but when it's between +19 or +20, getting other powers is desirable.
You could apply level limits like 1e/2e did, but this is generally an undesirable feature because you're essentially making it impossible for multiclass characters to continue adventuring with the rest of the party once they "cap out." You could apply an XP penalty: -50% to earned experience points after some level -- say, level 10. This would mean that the hybrid character starts to really slow down at some point, but this also becomes undesirable because the character's BAB, saving throws, and hit points will wind up lagging so far back that the character can't survive. You could rule that your hybrid class functionality only continues until some arbitrary level, at which point you "cap out" one of the classes and go back to just advancing in one or the other -- so you could be a fighter/wizard 10, but after that you are gaining levels, benefits, and class features as if you are either a fighter or a wizard, not both. This front-loads a lot of benefit but lets you continue some amount of high-level progression, but then raises the problem of, if you can do this, why would you ever be just a wizard? Get the hybrid levels at the low end for survival, keep advancing as a wizard at the high end to get the best spells. This is because of the difference in how progression works between 1e/2e and 3e/Pathfinder; in early editions your progression tended to top out around 9th-11th level and most progression after that was mildly incremental, while in 3e/4e/Pathfinder progression continues on a linear scale regardless of level. Thus, in 1e/2e having a maximum level isn't so terrible until you start talking about a difference of ten levels or whatnot, because the major feature improvements happen at lower levels -- having a 9th level dwarf fighter in the group in 1e or 2e isn't that bad when the rest of the party is 12th level, because that 12th level human fighter really only has a +1 or +2 over you on attack rolls anyway and doesn't get all of the cool dwarf abilities.
One possibility is to rule that each time you gain a hybrid level, you can pick the class features from one class or the other, but not both. But now you're back to essentially having half your level in each class.
If you stick on the 1e/2e limitation of putting classes into categories, and not allowing multiclassing of two classes in the same category, this does mitigate some of the stacking. So you'd have:
Warrior types: Barbarian, Fighter, Monk, Paladin, Psychic Warrior, Ranger, Soulblade
Divine types: Cleric, Druid
Arcane types: Sorcerer, Wizard
Vagabond types: Bard, Rogue
Psionic Manifester types: Psion, Wilder
(You can do your own homework by inserting your favorite nonstandard class here as appropriate.)
This means you don't have to worry so much about the 10 bonus fighter feats + all the class features of a level 19 <something else that rocks at fighting>. Instead, you're more likely to see hybrids that are trying to combine functionality of two disparate roles, but without quite hitting the peak of either. Still, this packs a lot of powers onto a single character. Having the spells of a 19th level wizard with the abilities of a 19th level monk is pretty crazy good!
Early 1e and (especially) 0e were even more stringent, and only let you combine a very limited schema of classes. This might do the trick, since you can basically eliminate any class that has a strong suite of class features. No multiclass barbarians, paladins, or monks! But you can stick fighter and wizard together and get something reasonably viable, especially because there's some collision of features (with a fighter/wizard worried about not having really good armor). If you do this, you could just enumerate which hybrids are considered permitted for multiclassing, such as cleric/fighter, cleric/rogue, cleric/wizard, fighter/wizard, fighter/rogue, and rogue/wizard. (Your top performer in such a case is probably a cleric/fighter, since the cleric gets to use all the heavy armor and fighter weapon proficiencies and cleric spells and bonus feats, while the other classes have some degree of collision over their use of armor with their skills or spells.)
Other other classes: This system starts to get really murky when you start adding on yet more classes. Since a 3e character can just take a level in another class, this presumably would allow you to advance as a hybrid some way, then add on something else. You could be a fighter/wizard 3, rogue 4. Doing all of the necessary computations would become taxing, to say the least, not to mention that this becomes yet another way to exploit class feature advancement in hopes of finding the quickest way to some favorable prestige class, feat, or power.
Multiple Ability Dependency: Arguably this is a feature, not a problem; when you multiclass, especially into two different disciplines, you make a character who now needs to have good ability scores in several areas. A fighter/wizard needs strength and intelligence to function competently in each class. This means harder choices about how to arrange your ability scores and more scraping for magic items to supplement them.
Where this ultimately led, of course, is in the single-class multiclass designs of late 3e and Pathfinder, in which two classes have some of their features mashed up into one base class, like the duskblade (something resembling a fighter/wizard) and the beguiler (a sort of rogue/wizard), both appearing in the Player's Handbook II for 3.5e. Pathfinder, similarly, has their new Advanced Class Guide providing hybrids like the warpriest (fighter/cleric-ish) and the investigator (ranger/rogue-ish).
But players still want to play their good ol' fighter/magic-users, so multiclassing in some form or another will likely never die . . . I'll dig into another variation on this theme next time!