Something I find interesting is units that do things another more common type does but in a different way. Examples : Ninjas and Rune Fencers, soldiers who heal, heavies who AOE, Bishops (extra range mages who part-time heal). Units like these add versatility when map slots are limited in some way, most commonly when ranged slots are limited.

Also if you have gaps in your lineup sometimes you are forced to use units differently to cover the gaps. I generally lack quality magic ranged damage so I use a Necromancer as a heavy hitting, better ranged Witch sometimes. Having Awakwened Sophie also adds a whole new strategy to my play as she works as a lightning rod, 3rd healer and burst heavy hitter.