yes I do like this idea of forest bonus best is offer a new type of unit (eg Cockatrice 3 strength with forest bonus +1 combat, forest terrain, speed 20, hero can mount, cost around 600 Gold, build time is 2 turn)
Actually since the Wolf is really the Wolfrider (Orc riding a wolf) it would make more sense if the Wolfrider:
A) Had Swamp/Forest move bonus instead of Hills/Forest B) Had a +1 stack bonus in the Swamp instead of the Hills since that's also where Orcs are from and have a bonus.
That then leaves the potential in the future for a stack bonus unit in the Forest, Hills and Desert.
As you noted the Unicorn already has a Forest combat bonus in Beta3 but I see no reason why the Elven Archer can't also have a +1 Forest combat bonus since that is the Elven Archers natural home.
We have thought a bit about mounting other units than flying ones. Haven't decided anything yet. Won't be anything like that in beta3. Could be an interesting upgrade for heroes perhaps.