Yes, it would take a lot of work… most will probably be engine work. It would need a few things figured out, such as the UI. How do you determine what layer is the destination of the move command.
Another thing is is it worth it? Will tunnels be too porky, where it has not only small passageways, but is cheaper as anti air and anti nuke is not required, Artillery may have issues when there is a ceiling, and super weapons may not reach it.
It’s mainly an idea. There would be menu problems to solve.