This actually sounds really fun because it encourages you to take the most tricked out hero you can.
And AoF has a ton of huge mounts like Dragons.
I don't know if the intent was actually Tough(12-15) Heroes but given the cover image I assume so.
Also because everyone will pick the biggest mount they are possible allowed (even without flying or fast) probably the two powers that makes you invulnerable and always win combats are to strong.
You can charge around units, so ironically it feels like the attacker is going to have to do everything he can to lock down the Ascended - defending - so it doesn't reach his zone. But he literally can't win combats until he takes 3 out of four objectives, that'll be difficult.
If both players had their own Ascended/something similar and could damage each other you'd have a Havoc Hero on a Dragon vs Imperial Hero on a dragon duking it out, or a Lizard T-Rex vs some Zombie Dragon with a Vampire or similar. Which granted, if they were the same the theme would be diluted, but I really like the image of it in my head.
I think you'd need to playtest this a bit but could probably make it really cool.