I think we all know about the infamous Subspace Emissary and how... tedious it was. A lot of levels had no significance on the plot, the plot itself had a lot of BIG plot holes and then there is the infamous Great Maze (ugh...). When designing Fan Disagreement, one of the things I wanted to do was redo the concept of SSE because to me, it has potential but was poorly executed (like a game I would rather not review *inset bad memories of that f^%$*&^ billiard puzzle...).
For this idea, I would like to divert your attention to the LEGO games, each of the levels in those games can be played and completed by any of the characters, like how SSE was trying to do, but in the games, the more characters you have, the more rooms you can get into in said levels and the more collectables you can find. This is pretty much the idea I would like to implement into SSE. Each character would have a ability that is shared between other characters and, depending on what characters you bring into a level, the more collectables like stickers and trophies you can find.