Okay so basic premise here is that rangers are kind of gimped now and could use a buff.
Suggestion:
Everytime an account reaches X kills (X to be determined later) of a specific mob type that mob is then flagged as a favored enemy, but does not count as an extra favored enemy beyond counting as one if the player does not have any favored enemies already. This mechanic would use the monster manual volumes to track this and require different manuals for different mobs.
Essentially this would expand the effective mobs that rangers favored enemy skills work against in a manner that ties nicely into mechanic that is oriented around killing mobs of a specific type. For rangers it would grant more benefits then others (more damage per hit and possible enhancement increases).
Also, adding the epic feat bane of enemies where every weapon you wield works as a bane weapon against all your favored enemies would be another nice idea for ranger buffage.
Note, this would be a non-trivial increase to ranger dps against less commonly seen mob types but have no effective boost to their dps against most end game raid bosses.