Hm... let's see...
Warchanter was, by far, one of my favorite bard PRC's. Period. Perhaps something that allows at least bardic music to continue advancing while multiclassing, or a way for bardic music to be good while doing so.
More caster feats would be nice, too. Arcane Strike and the Arcane Spell Failure reduction feats were a start, but I NEVER like Metamagics (save for the low-cost ones). Maybe something that allows arcane spellcasters a greater variety, say... Giving a Wizard a feat that lets him get an extra spellbook that would be stored in an extra-dimensional space and can only be summoned when the previous spellbook is lost or destroyed, or something that grants the Still Spell feat to all bonus spells a Witch gets from her familiar without increasing spell level.
Some +0 LA templates would be nice. Things that make a race aquatic, or subterranean, etc., would really allow for some interesting concepts without having to introduce dozens of new races.
Along the previously mentioned Warchanter line, perhaps a way for Bards and maybe Summoners (When Published), as an alternative class feature, to get rid of spellcasting capabilities (or inhibit them to, say, Paladin/Ranger levels) in order to gain an increase to combat capability? Say... trade the spells for full BAB and maybe... a bonus feat every 6 levels, starting at level 2? It definitely wouldn't put the fighter out of business, just make the classes more feasible for melee combat, especially considering that Bard is as versatile as it is.
Food for thought, I'll keep thinking about some of these things.