So yesterday Nintendo announced red coins and keys were coming to SMM. Both allow for doors which cannot be opened until they are unlocked. The pitch is for allowing more structured levels where you have to collect stuff or kill stuff to proceed but I'm thinking they could be viable simply as collectibles.

I'm making an 8 world saga soon and if I could use there coins and keys as kremcoin and DK coin proxies thoughout the 32 levels that'd be really cool I think. Only concern I have is that if they don't unlock anything people will be confused. If they do that's an extra door and level space that needs to be used in every level which could be detrimental if I'm using doors to reset things. Then again a "hidden door" could be a cool thing to hide in every level. But does that read as secret door or does that read as non optional door which ll frustrate people?

What do you guys think? What do you makers plan?

Having you collect them to access a secret area might be nice, but it depends on how good the 'secret' is or, like you say, some might find it frustrating. Unless you can somehow make them lead you to a secret exit which is either easier or cooler in some way. You can now add a mandatory boss battle to the end of each of your worlds.