Unlocked character customization would be a Big undertaking, almost too bothersome, you have something like that with the laboratory already, letting you add some mods and what not, so maybe an overhaul of the laboratory system would actually be easier to implement, but from the get go you would need to sink into the depths of coding in GDScript and trust me, takes time and patience. I myself am not even sure how I would even begin tackling that lol.