One thing that's pleased me no end is the fact that there is enough randomness that you can never be 100% sure which combo would be best for any given game. Some pairs, like gas/hydrogen, have a strong tendency to be better than others, but nothing is guaranteed! The Last game I played as gas/hydrogen, all I seemed to be finding were gas/methane.
That makes sure the other options remain valid, and that's the key to a long life for a game!