In short, i am thinking about ordering some pizza
so do you want to code this or just asking for volunteers to do this in detail explained idea?
Well, not sure if i can code that, but i can draw some art, and i have an idea how to make aerial ship tileset from mixed ufo defense/tftd assets. And about details...
Well the base either have several different layers to build (and defend). Basically it may be a flying sandwich from 2 or 3 bases. I guess that may be easier to do that than expanding current base-bulding space limits. And additional decks may be available after some research and spending.
Must have some new critical components, like bridge, propulsion and reactor. Need an appropriate art for that. More modules may need more reactors, and more decks - more massive and slow to accelerate base become. Enemy may not attack all base levels, just those critical components, so there will be no potentially exhausting 3+ levell missions...
Base defense modules can be used as weapons in combat. Though this can be OP even if base itself will be slow to move and accelerate. There can be only two weapons, or many modules assigned in batteries (OP again). Either way if enemy ship comes too close, base defense mission starts, so gravity shield can be very useful, as it may act normally or be an actual shield, so enemy will have to wear it down before trying to board your base.