Gmod sorta already done that.
gmod = shoot a wall of cinderblocks and it falls
roblox = shoot it with tiny noclipping balls that do damage.
gmod = use hoverballs and thrusters on a thing to make it fly
roblox = free models
gmod = build you own vehicles and weapons
roblox = try and build your own vehicle without having control over whichever way it goes.