Everything on one stage would be impractical, especially since I want to include a time attack mode.
The emerald idea is a good backup plan, but I think I’ll try the emblem idea first. With emblems, I can place multiple emblems in one stage. If you’re interested in the details, I plan to have one emblem unlocked by having a certain amount of rings at the end of the level, one emblem unlocked by having beaten every enemy in the level, one emblem hidden in the stage, one emblem at the end of the stage by default, and one gained by completing an end-of-level ‘bonus stage’ (which also awards points for completion, so there is a point to doing it even if you already gained that emblem).