First of all, I experienced the same difficulty progression with a mage on hard. It was sometimes challenging in the beginning (I had some minor losses in a few fights, usually against heroes, and was too lazy to optimize them for no losses), but starting with mid-Verona, difficulty dropped rapidly and the battles became a joke (exceptions are bosses and maybe some heroes who cast "dangerous" spells. Crazy miner with mass haste can be annoying, armageddon casters etc).
Anyway, fun armies from my experience:
- A mostly lizardman army can be refreshing to play (if you have access to Reha...). They have various interesting units: Gorguanas, Highterrants (or is it Hayterrants?), Brontors (Charging across the whole battlefield can be really satisfying), Gorguls (Hitting two units is nice, and the extra turn ability can be funny. I had them act four times in a row one time with my warrior. Plus, if you have a lot of them, their stats are very good), and maybe even Tirexes and Choshas, but I never used them much (Gorgul damage outweighed Tirex damage by far with a late game warrior)
- Demons can make a pretty solid army as well that may play rather differently than your current lineup. I used them in KB:TL for the dragon labyrinth (archdemons are a dragon's worst nightmare)
- I could never warm to a dwarven army, but it may nevertheless become powerful if you get the dwarven items - royal hammer, horn of king borg and axe of ice should make for a rather powerful army. But as mentioned before, they always struck me as somewhat boring...
- Single units: Royal snakes are a good choice in most fights, especially if you have a snake ring (which makes them awesome damage dealers... I remember one playthrough of KB:TL where I got two snake rings. Needless to say, the resulting snakes of doom were part of my army most of the game). Archmages, royal griffins and demonologists I find hard to go wrong with either (summoner makes them even better), but all of these units will likely not make your battles much harder
Finally, you could try the Army of Light/Darkness mod if you want new units to toy with, but I find some of them quite unbalanced... (ice nymphs are overpowered, as are magi of the light, just to name two examples)