Isn't that covered with the self-serving role you are able to play?
A driven storyline based purely on "evil" intent wouldn't allow room for people wanting to play the hero. It would be a niche premise that only some people would like.
Not true. There are games that even BioWare has done with evil endings just for those players. KotOR is an example, so I can see that happening. However, it's unlikely to in a DA game. Given that there isn't really even a good/evil, paragon path in the DA games, it's really more about how you make your decisions.
And I wouldn't see what a good-aligned Inquisitor is doing just for the sake of power and personal gain. In most games with this system, the darker or "evil" character generally gets things done via brute force. They are more after what is in it for them, and what power/prestige they can achieve. For them, power is the end all be all. An evil character doesn't care who or what s/he hurts, as long as they get what they want.
For a good aligned character, unlimited power isn't generally a goal. .A good aligned character usually uses diplomacy and other ways of making nice. Many times, they aren't saving the world for themselves, they are saving it for everyone else. They are also more likely to choose an option of self-sacrifice rather than harm others.
Hell, as an example, my dwarf isn't going to want a damn thing to do with this. She doesn't want the power, she doesn't want the responsibility. She's going to be a very reluctant hero. As time goes on though, as a good character, she's going to come to realize that it's up to her to unite the armies and save Thedas. She doesn't care about being a supreme ruler, or having weird powers. She wants to get her job done and hopefully go back to a life without being forced to be a ruler for the rest of her days.