Beat sts1 with all 4 characters. What to do next? Don’t really have motivation to just do it again. Is there another goal?

Beat sts1 with all 4 characters. What to do next? Don’t really have motivation to just do it again. Is there another goal?

I also don’t really care to do the game modes that’s just the same thing but harder (although idk how to even activate that), that never really appealed to me. If that’s all there is left to achieve then I’ll move on to the second game :)

Thanks!