I think I want to make a world where the elves are the rulers. After all, they live for hundreds of years. Just by personal connections alone, they should have a huge advantage over humans. I also don't believe in demi-human level limits, so this seems to be a natural extension of what that rule change would mean.
It seems to me that the Melniboneans in the Elric Saga by Moorcock would be one potential model for the elven rulers. Another could just be how Galadriel is portrayed in the LOTR movies. Both are mysterious, and quite alien to human psychology. I know I don't want the elves to just be super-powered humans. I want them to have a distinct feeling from the other races.
I might have them be slave-owners. "After all, why should humans, who only live for maybe 60 years, be treated any better? They just act like spoiled children unless they have elven guidance.", says the typical elf.
I don't really want an elven Dark Lord, though it is a possibility. If normal elves have a ruthless enough feel, then a Dark Lord would be redundant, anyway. Maybe the crumbling elven empire on the edge of the campaign map would be enough, with the actual campaign area the wilderness next to it, or even a province that has revolted in the not-too-distant past.
I'm actually a bit surprised that none of the major published campaign worlds have tried a setting where the elves are the big movers and shakers, and everyone else is just living as their servants or in the wilderness that the elves don't care about. It seems a fairly obvious setting hook to me. Instead, they generally go with "There are only a few elves left in the world, and they are tree-huggers who never leave their forest". I did like Eberron's ancestor/undead worshipping elves, but even they are no major force in the world.