Always liked the idea of pseudo-Invoker like unit. It doesn't really have to be a hero. Basically using 2-3 resource type Abilities (typically "orbs", but can be ie. "cards") that must be combined, and then redirect to a new powerful, and by giving some information, wanted ability. Maybe not even a wanted ability to use, but an instant effect upon correct combination. If it features 3 or more resource abilities then order of combination should not be taken into account, unless you are up to make n^n abilities. With 2 resource abilities, order of combination makes sense because otherwise you would be able to make only 2 effects.
A tinker like unit (not really a spellcaster but the concept could be tweaked enough) that collects scrap metal to activate abilities. More metal, more abilities/more powerful existing ones. Scrap metal should just an item that is produced passively/actively by the building that trains the unit. The player can then choose to distribute it or asign it to one unit.
Maybe one of the abilities the tinker unit has is precisely a factory to produce metal, hence certain units can become a self sustaining unit.
An alchemist like unit that periodically checks nearby area, randomly generating 1-2 plant items (could be just dummy units, the thing is that you need something that can be ordered right-click). Upon creation, plants are removed after some time. Basically the unit has to choose the ingredients, and these are periodically refreshing. Upon selecting the 3° one, an "alchemical like spell" is given to the unit.