Split-Upgrade tree / buildings for existing units
The idea comes from Disciples, where you choose a path for units. That could be a way to get new exciting units, with special traits, or even change their function.
Like the Rana hunters, when upgraded to storm guards the next path could be a split upgrade (only one upgrade path can be taken) - one path would grant them lightning javelins to throw at enemies, the other path would make them equipped with heavier armor and shields.
The Crawlers could be upgraded to one path giving them a ranged poison spit attack and the other path could lead them to gain the ability to burrow into the ground (gaining 50 defense) and the round after appear in another tile dealing damage to the unit next to it.
Should be researched to unlock the path, so it counts for all buildings on the entire map or could be on building basis, so you would have even more options (just not more units)