If we make unlockables, we should have something like a Nightmare Mode and a Boss Rush mode. The idea is that both could be completed upon reaching special conditions, fore example, the boss rush mode could be earned by collecting all seven chaos emeralds and completing that game 100% by traveling through every zone. The Nightmare Mode could be unlocked by completing the game any % without collecting a single ring. The idea for the nightmare mode is that it'll be much harder, but much more forgiving in some places (Sonic starts out with all seven chaos emeralds but the ring cap is 50). There could be new level layout designed to be harder without using lame speed traps too much, and there could be new mid and end bosses, reaching up to be screens or tens of screens high. And when you beat Nightmare Mode, you unlock something really fucking special (as opposed to just special) since the challenge is designed to be akin to beating a bullet hell shooter without dying.