Here might be some of spirits or demons or elementals? :
one that gives small amount of regen to player (of course doesnt stack with bard song)
one that procs at mob from time to time - maybe slow or silence
one at avatar levels that can give like 5 spell bonus or spell bonus + heal bonus at highest levels
low level version might boost one stat for time its being summoned(it contradicts with spell stats being avatar spells, but you cant benefit from other summons when you have that one)
one that might reduce monster procs penalties, affecting just a summoner
one that can affect room like casting inferno(or is that one cast on mob?) or fog and such
one that can search for caches? telepathy spell is really useless

and so on...
As bards are close to mage class in terms of spell casting, maybe can give them that ability too, probably with shorter range of summons or just different summons.
Truth, greedy little teapot