This is some juicy stuff - a cleric PC has latched on to a power source that gives real, tangible benefits (spells! turning undead!), but it comes with a philosophical cost. You aren't out shilling for a particular god or ethos or pantheon. Instead you're advancing the cause of a material concept, some kind of humanoid interpretation of a chunk of the world. This means you're stuck between trying to fulfill your personal agendas and emotional needs and community demands, and also trying to interpret "what is the will of Fire" or "how do I become more like Earth" or "who would Water support in this conflict." These create strong role-playing opportunities for clerics. Whenever a cleric approaches a conflict, they should not just be thinking about how they, personally, feel about it, but also about how it meshes in with their abstract concepts of the elements. Some clerics will be very practical about this; others may take on a philosophical bent, looking for how an element symbolizes certain things - fire as inspiration or cleansing, water as malleability or purity, air as illusion or freedom, earth as stability or secrecy.
Beyond their philosophies, though, clerics are keepers of real power in the DARK SUN setting, and that's always an attention-grabber. As noted in Earth, Air, Fire, and Water, many villlages and tribes in the wild have shamanic clerics to assist them in life challenges, whether with healing magic, divination, defense, or just general wisdom. In the city-states, though, it's a different story. Clerics are a threat to the status quo. Normally it's the templars who have the power to invoke "divine" favor. Other people who can do it without calling upon the sorcerer-monarch? Heretics. Pagans. Whatever "othering" religious terminology the templarate might choose to invoke. Even if a city-dweller needs the help of a cleric, the fear of being found out by the templars - of being branded as an enemy of the sorcerer-monarch - is pervasive. Clerical cults in the city-states must perforce be secretive, or else have an alliance with a templar patron. Those who truck with clerics risk denunciation.
Of course if you're rich and you have a little prestige of your own, maybe you can hold that denunciation off. Noble families and merchant-houses might like to have a pet cleric - someone who can work elemental magic on their behalf, without being beholden to the sorcerer-monarchs. An influential patron might protect a cleric from such entanglements, but this too comes with a cost. The cleric's movements are no longer free; working for someone who puts this kind of political clout on the line means spending your time making good on their investment. And it also means that clerics are constantly under threat from these groups. Why would a noble family pay a cleric and protect them from persecution, when they could just purchase the cleric at a slave auction? All too easy to pull a few strings, bribe a low-ranking templar, and have the cleric hauled off in ropes, just to turn around and buy them at the block later. A horrible fate, but all too possible for someone who has the power to create water by magic!
This is before even getting into the notion that clerical cults in DARK SUN don't necessarily have to be benevolent. One would hope that PC clerics would be heroes, but any clerical cult could be headed up by an evil leader - someone who's gained a taste of power and wants to use it for control, or revenge, or wealth, or general mayhem. Just because a cleric can conjure water doesn't mean that the cleric is guaranteed to be a good person. For PCs trying to survive in the city-states, running into a clerical cult run by an evil mastermind can be quite a challenge - since the PCs are often primed to think of other hidden power groups as enemies of the city-state (and they may well be!) and thus as natural allies, while such a group could turn out to be enemies or, worse still, a group that feigns friendship in order to use the PCs.
The ultimate end for clerics in DARK SUN, of course, is elemental transformation (well, for some clerics, anyway) - literally becoming an elemental. This raises interesting questions: How much does the cleric retain their former humanoid sensibilities? Do they eventually become as alien as their patrons and stop caring about humanoids, or do they become a "person in elemental skin" who is now agitating the cause of their former friends and allies and family from inside the elemental hierarchy? Is the cleric still aware of the need for a balance of elements to keep the world alive, or does the notion of overwhelming the world with one element become too strong a call to resist? As with all endgame transformations, this is ostensibly something that is changing the character and potentially changing the world at the same time - in this case, introducing the idea that there is a bridge between mortal and elemental, more than just an arrangement or a power-brokered deal but a true merging of philosophy and flesh. What will the cleric do with this transformation? Take over part of the world and try to turn it into a reflection of the elemental planes? Bring humanoid sensibilities and ideas to the elemental lords and perhaps upset the stagnant conflict of elemental opposition? Throw open the doors of the elemental cult to others and form a new society fueled by elemental powers? Each of these possibilities creates ideas not just for endgames for clerics, but food for thought about the relationship between clerics, elementals, and the common people of Athas.
So your cleric is more than just a repository of spells and elemental powers. They are a doorway, a psychopomp, an intermediary that goes both ways: Bringing elemental power to the humanoid world, but bringing humanoid ideals and motivations ot the elements. At the same time they underscore that all greater powers on Athas stem from some flawed or corrupted source, be it the false divinity of the sorcerer-monarchs, the deathly temptations of defiling, or the nonuman elemental and natural spirits - all offering power, but not safety, community, or answers.