I'm fine with the demons we have; some model upgrades (like the recent druid forms) would be nice... but WAY down the "to do" list to ever really see the light of day.
One thing I would like, however, would be the ability to give my minion some gear. Say, have three "equipment slots" - the actual slot type to vary by demon - that you could just equip standard dropped gear on. Who wouldn't like giving their imp a spell power trinket? Or giving their fel guard a nifty new axe?