Simply an idea for the, as far as I know, un-used 400 favor for House D,P,J,K. Maybe even other factions.
Basically, unlock, say... 2750 total favor, all toons you make on your account start with 1 extra level 1 feat, with feats selectable based on who you have 400 favor with, or whatever number seems more balanced.
For example, 400 favor with house J would unlock the "Member of House Jarasco" background feat, which gives a +5 stacking bonus to heal, and a 50% discount to all house J purchases, and a +5% xp gain from House J patron quests, and +1 loot from house J patron quests/rewards.
Or just one or two of those abilities.