I think it would be too easy if you had a creature / minion with you at each level. I had a level 10 vampire in most of the maps - and some maps I defeated in just a matter of few minutes, because I just went straight for the opponents dungeon hearts - even if they had loads of creatures, they could not kill my lvl 10 Vampire.
So I don't think it would be balanced if you can transfer your creatures often.
I kinda miss maps where you start out with 1 creature, no imps or anything. I like them, they take time to win, and they are challenging.
Also the map in DK1 where you can't build a training room, untill very late in the game - is very nice too. And then imagine, no training room at all.
About the gameplay - I think it could be cool with the three planes, or what you call them, upper middle and lower. where the middle could be some barrent place, where you only can build traps and such. (AND you can restrict your imps for going there - so they won't get killed over and over agani by lightning traps! - and there should be a creature, which can disable traps, I think)