I really want to see a new kind of linedef flag called "No Other". Basically, it means that the linedef will not function if the character is a skin other than Sonic, Tails, or Knuckles (or at least has an ability set different from those three). It would make it significantly easier to make levels which are more forgiving and flexible towards addon WADs, not to mention potentially enabling mapmakers to create interesting secret areas only accessible with addon characters. This might serve the additional purpose of making the creation of custom characters less pointless.