I played War of the Gods for about an hour a couple of years ago. And I suggest not just an eighth-level unit, but an ultimate super-duper unit, like the Dragon King in the fourth game, only even bigger and tougher. You could make it a choice: either have this amazing unit or a normal army. As for the balance... in King's Bounty, people struggled with the bosses.
Although, it's a silly idea that probably won't be implemented.

And double-check the fourth game, seriously. There's enough good stuff there for ten games.