RPG Brainstorming

I've thought about this for a while and if you've gotten around to playing XCOM Enemy unknown or XCOM 2. Would you suggest the AP system that's present in Fallout, Silent Storm, Wasteland, and Jagged Alliance? Or the 2 turn system with room for more turns from say something like XCOM EU? If say me and a team get together to make an RPG? Me and my friends have been wanting to do this for a while and we need some ideas and input from people of a few forums. I've been thinking about this idea for a while, it'd take both inspirations from Fallout and XCOM. Though set in more modern times and companions are an option but I want to make the game as flexible as possible with room for every characters improvement stat wise and I want to take a system that isn't limiting.