[@Genbor]
I like the third option ,it would be fun  to play a DnD campaign with  an achievement  based progression )

I have a suspicion that our game will be more about plot and RP than grinding for levels.

Our class will probably determine our characters' abilities ( who can sneak, cast magic , shoot etc)

But again I am not a GM ;)  

For all we know our campaign might be an endless dungeon crawl XD