See, I'm being torn between two different things here. I like the idea of no system, but then again I don't know how to do XCOM without a system. If I use a system, I don't know which one to use.

Without a system it would just be a case of letting the players act like soldiers, albeit heavily armed soldiers with psychic powers and gene modification, or giant power armour suits.

With a system you could just ask players to create skills of their own, as is happening in another RP I'm involved with. A character would be able to choose whether to use a standard attack, or their special ability each turn, as well as moving and/or taking cover.

Players start out with only one skill, but then gain experience for each mission they survive which could the be used to purchase more skills. Each player should then end up with a unique character, perfect for their style of play. Also that's something which couldn't be done anywhere but in a roleplay since other games are limited to their own systems and wouldn't be able to give that level of customisation.