Let’s come up with a monster.

Oh, you're suggesting some really cool monsters, level 6-7 at the very least. But who will be available in the early levels for each faction? Here's an idea I had a few months ago for WOG 3.59: Desert faction - unit: Tumbleweed. No higher than level 3, infantry, but with very high speed (up to 12). It attacks the enemy, can paralyze, poison, or entangle it (like a Dryad). But it's light and quite vulnerable. In an upgraded version, it could gain a retaliatory attack. Its speed and "talents" could compensate for the lack of archers at the beginning of the game.