What I'm looking for is a way to make the game interesting for the player after those quests have been finished. The monsters are now buried, what is the character going to do now with his own life (appart from maybe getting married)?
But that could be said about anything that isn't infinite. At some point, you have to either add some game elements that are infinite (e.g. creative features like in Minecraft and similar) or keep on adding bigger and bigger scopes (WoW, anyone? ) until you decide that there's no real need for more, because players won't bother progressing that deeply into the game before moving on to a new game.
I think the most important thing any game developer can do in this regard is to just add new core elements to the game - I.e. new dimensions of gameplay, as opposed to taking an already existing dimension to new heights. The problem with being too linear with the game content is that, if a given game feature lies too far into the future (inaccessible to newbies), then fewer people will end up enjoying it because they may very well get fed up with the game before that time.
One example for me was Age of Conan. I tried it till level 25, and then I got fed up. It just felt too repetitive. But then some people said that I should get to max level because that's when the fun starts. Well, I'm not going to play a game where I need to do a million different boring things just so I can get to the fun part. So I quit. A game should be fun to play when you play it. No excuses.
I think proper progression in a game is important, but I also think a lot of people overestimate that importance and let it get to their heads. One way of solving progression "bottlenecking" is to provide players with additional dimensions of progression - something that is available to everybody, regardless of their current level of progression. This is what is sometimes called "horizontal progression".
A good game should have a nice balance between horizontal progression (n specializations) and vertical progression (the depth of each specialization), IMO.