While a very interesting concept, I don't think it'd work because of the camera being blocked when you go underneath a lane. That, and using an ability that has to be targetted on the ground will be impossible on the bottom lane.
It's not that hard generally speaking. Lots of games have 'occlusion' view - by lack of a better word, english is not my first language so please tell me the correct term for this - where objects on the front will become transparant and you can interact with objects (characters, the ground) in the background. It's not that big of a problem per ce, but Demigod's engine would probably need some additional code, which I don't think GPG is willing to write.
Yes, making maps is hard and time consuming, but that's what you have coders for. Like I said TSA will be a great addition in the creative department. You have coders, other people, to actually write the code for it. The coders will tell him if his idea is viable or not, the beta testers will tell him if his maps work or not in the end.
But this is all hypothetical. He's not working for GPG. He'd be a great map modder, I'm sure, but it all depends on wether GPG is willing and able to make good enough mod support for creating maps.