Honestly you could probably make drastically different move sets for each of the Snakes if they wanted to.
Big Boss could focus more on the survivalist aspect, maybe even get a self heal ability and the ability to throw animals at people.
Venom has all sorts of crazy gadgets, probably they could just focus on the fultons and his
"ROCKETO PUUUCH!"
Since old and solid Snake are the same individual you probably don't need to separate them. He'd probably be the base line fighter guy, maybe really focus on his Socom tranquilizer gun.
Liquid always been more of an acrobat and brawler, would probaly make a good rushdown character in comparison to the other Snakes.
Liquid Ocelot would probably just be a skin for Ocelot,