Yes, both things are entirely possible. We even had controllable Ornithopters in earlier releases. You can make a map and edit the rules to play custom games in multiplayer. Look at an existing https://github.com/OpenRA/OpenRA/tree/bleed/mods/d2k/maps on how to apply your own rules and https://docs.openra.net/en/latest/playtest/traits/ on how to use them.