I like #4 as well, and maybe instead of achievements there could be special items that drop randomly and are then used to 'craft' (for lack of a better term) a new playable character. Good luck! :)
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That's kind of what I was thinking. Like you collect a certain amount of crafting materials for each new player character and then make them (summon them?). Anyway, you could make different materials more or less rare so it would generally be easier to unlock lower level characters, but there would still be some amount of randomness and/or planning behind which characters you unlock first.