The main things I think of are different helpers in the vein of the clones, but each has a different specialty they are good at. The primary thing I think of when contemplating replayability is, for example, if there was a stun based helper, would I want to replay the game and try to go for a build that is mostly stun helpers? If I can replay the game and live out my goal of using mostly a specific upgrade, that might get me to play again. But you want to avoid making everything too good or too simple strategy wise. It's a complex topic.