At this moment, the only support unit in the game is the monk. Well, you could consider the siege tower and transport ship, but those are for transport.
The monk has 2 functions. He converts enemy units (attacking them to convert them), and he can heal allies (attacking the allies to heal). The monk has value, without dealing direct damage.
What if the game had other monk-like units, even as a unique units? I think they should be considered as “monks”, so scouts have bonus against them. I have thought in the BARD (or commander, merchant, whatever), or something like that. He has faith like monks, that is depleted with his abilities. If you target allies, the allies receive some direct buff (more damage, armor and maybe speed). If you target an enemy, the enemy receives the opposite nerfs (less damage, armor and slowed a little). The bonus shoudnt stack with itself (to avoid making a champion an immortal warrior). Or maybe it could stack, but only if the bonus acts as a percent bonus, similar to the hill bonus.
Other possible bonus and nerfs this unit could provide: make units weaker or stronger against conversions, increase the range of the unit by one (even melee units), etc.
To make this work, the monastery technologies should affect the unit, but that brings complications: what about units more resistant to conversion? The effect of the songs are only active while the bard is targeting the allies or enemies, not like the conversion, that is triggered after a while. Maybe the monastery techs related to conversion shoudnt affect this unit.
Is this unit useful? Depends of the strength of the bonus received. You could slow some paladins while speeding your pikemen against them, that could be useful.
Other possibility would be making the bonus automatic in an area around the bard (and the bard couldnt attack anything, he could only just move). You can not stack multiple bonus from multiple bards. Being an area bonus, this could work even if the bonus is small, as you can buff many of you your small units at once. Or, instead of an unit, this could be a new kind of building that acts like a totem. Maybe a TAVERN. This building could be placed to strength your units in strategic places. It adds an interesting new layer to the game, as you may want to focus first the bard/tavern like you want to focus monks.
Opinions?
Some questions. Could you use a tavern in offensively (placing it in the enemy camp)? Would it make more sense to just make an unique technology that makes your Town Center buffing close units instead? What kind of building would provide nerfs to enemies? A graveyard?