One issue for any game, to me, is consistency of tone. Will the PC's be Boy Scouts out to help the world? hard-bitten mercenaries out only for their own best interests? Machiavellian schemers who value nothing but their personal power? Having a Boy Scout Paladin in a team of hard-bitten mercenaries can't hep but create in-party conflict. A discussion of campaign tone sets the stage for compatible characters.
Similarly the game world - if you want altruistic characters, then screw them over every time they behave in an altruistic manner, they're not going to gleefully continue being altruistic. If "being a Paladin", or "being Lawful Good" means "having to make suicidal choices", no one will play a Paladin or a Lawful Good character. Do Evil characters get to do whatever they please, without consequence, or do they get hunted down by forces of Law and Good? Big difference in choosing to play an Evil character.
If some players expect a black and white morality in the game, with Good and Evil wearing white and black hats, and others expect a morally grey world, someone will be unhappy with the game. So discuss the morality up front.
If the players just want a PvP game, then maybe they don't need a GM at all - they can just send an endless succession of characters into cage matches with no need for any outside world, NPC's or GM.