i am also working in that field (levels, items) on a project; that is to say i'm designing things and a brave coder plans to implement them
as with smoths' work, yours will likely be picked over and analysed as well, should you come up with something good

(and frankly i'm convinced you will because everything else in cursed is money)
just to add a few to knorkes ideas which are all cool;
- a teleport spell that allows you to move your hero to an advantageous position near an allied unit every few minutes (for larger maps or huge battles to possible offset a relatively slow movement speed)
- "summoning" abilities to build a costless tower or mobile unit on a countdown near the hero
- temporary barricades (i.e. anivia from LoL)
- capture (brainwash? sapping a tower? it's been in a lot of RTS games, i enjoyed using spies in C&C generals to steal labs and plants)
something of note... in my very very early wip game plan (action RPG) abilities will all be from picked-up items, allowing players to customize and combine different abilities on their load-out to make the most out of their characters' strengths or the NPC enemies weaknesses... you could just have a shared set of abilities for cursed and let the players pick a new one or an upgrade every few minutes... would certainly open the idea of heroes up strategically, and as long as you can avoid noob traps, sharing abilities for heroes across the two armies should make balancing comparatively easy and let players pick abilities that apply to the map (nobody would pick the swimming guy on DsD etc)