The current favour rewards as far as feats go are pretty pathetic at best.
They don't encourage players to complete content that isn't usually played on the live servers, except when it comes to hardcore for the favour tables.
If there were favour trees, you could implement small, but stacking bonuses. Nothing game breaking... maybe some of them containing +1 to a particular DC type or a few extra hp at the top end of the bonuses. Oh, and you cant fill a tree without max favour. This could be adjusted so that once you got to a tree with 5 tiers you could just start increasing the cost of certain enhancements based on how much extra content is added to the game, rather than keep having to add new ones.
I dunno, maybe its a stupid idea.