I would like to see systems like the FFXI Mythics and Empyreans. Beat a substantial portion of content (like the entire ToAU expansion) and have a Holy Grail at the end. One nice thing about the Empyrean model is the fact that your weapon is upgraded in stages, so you have something to show for your progress.

I also dislike a heavy reliance on currency. It may well be the only way to control how many relics get into the game, but it takes away from their appeal if they essentially have to be bought.