If anyone has played Stalker: Call of Pripyat (being an open, free-roaming FPS I think a lot of TES fans have), you know what I'm talking about.
I'd like Skyrim to expand on this. If you finish a long side-quest line, you get an achievement, but not only that. You are awarded with a gameplay element that makes sense in accordance to that.
For example, you finish the Dark Brotherhood quest line - you unlock a new, or improve an existing sneak attack mechanic. Or you are able to summon (LEVELED) assassins to help you out. Or you get some other benefit, but in turn get a penalty or are attacked on sight by certain individuals/factions.
I think that sort of thing not only adds meaning to otherwise useless achievements, but also adds more flavor and diversity to the game.
Opinions?