Floating buildings

I was pondering as I was looking at the elixir ccollectors. What about building upgrades (or buildings that start this way) as floating buildings, untouchable by ground based, ground-attacking units? And I don't mean every building, but some that you absolutely have to have a mixed army in order to 3-star?

or maybe:
choosing and upgrade path for some buildings, such as when your elixir collector reaches level 11, you can choose to upgrade it to an aerial storage, but it doesn't increase storage amount by as much (up storage amount by 25% of what a regular ground based upgrade.

or regarding an defensive building, the damage doesn't go up by as much with the upgrade (it normally goes up by 4, but if its upgraded to an aerial building, the damage goes up by 1)

Adding to the idea of floating buildings, maybe buildings that while floating, maybe there's a building that auto protects itself by moving away from attackers? (Opposite of the barbican kings movement for example)