Dual purpose units that have identical offense and defense (and would benefit from either off or defense boosts and hero equipment etc)
They don't need to be powerful, so long as they have a short enough build time and cost equal gold and iron to build.
Cheap and cheerful, and available from low levels, so would need inf/cav/occult options
Zero food consumption would be a good idea too (or very low), and it could help in bg if they had a very high food cost to build.
I would also like to see more Eldrich units, and the ability added to darken more of the Amulet units
Edit: As an example, these dwarven units are almost worthless for many reasons, but a similar set of units could be added with 120 offense and 120 inf/cav/occ/best defense, be classified as inf, cav, occult and bestiary (4 different build queues) and be given better travel speed, build speed and much lower food consumption etc (or upgrade the existing units)
http://prntscr.com/fthq56