That could work, but it would require quite a bit of rework on the game engine. Right now, turtles are always camera-centered vertically, except when they first spawn. A lot of mechanics rely on that, like falling sand physics and ice sliding, so changing it would introduce new bugs and require a good chunk of reworking. But technically, yeah, it's possible.
As for swapping positions with another cell, if it's on the same row as the turtle, that's more feasible and would involve fewer engine changes. But once we go into vertical swaps, we're back to the same complexity as digging upward.
That said, I'll keep this kind of effect in mind for future turtles. For now, Nix is planned as the final turtle, at least until the game hits a more stable 1.0 state.
As for Caelo, he's always meant to be floaty, but not quite flying. I've always seen him as this goofy, slightly clueless angel turtle. The clouds could work well as a little Easter egg in the future. I once had this idea of cow-shaped clouds spawning after cows were defeated, but it ended up feeling more disturbing than funny, so I scrapped it.