Incarnum. Totemist was great. Bring everything up to that utility level.
Tome of Battle. It was a brilliant first draft, but it could greatly benefit from a revision and a lot of expansion.
Skill Tricks. Some can be pretty decent, but after playing one or two skillful characters, you've burned through most of the decent options (and thus most of the novelty). I'd like to see more interaction with Maneuvers, which can also feature skill checks -- and interaction with Luck feats, which seem like a good match for a skillful character.
Spells that care about skills rather than just boosting skill checks, or making skills redundant entirely. The lorecall spells had a few interesting ideas, but there's a lot more design space in that direction.
Cohorts as a consistent system, rather than the hodge-podge of feats and spell effects (e.g. animate dead and planar binding) which give you easy minions.