My biggest desire is to use future-gotten ghosts on previous levels. When I go into the Time Vault to replay a level, it only lets me use the ghosts I had at that stage of the game when I initially completed that level. I'd love for it to be more like the Lego games where future level replayability is baked in right from the start by having things you cannot do until you progress a little in the game, gain characters with new powers, then come back to the level to play again. Mostly, a lot of fun ghosts weren't available until late in the game and so I can't play but a level or two with them at all. Partially I'd love to bring them back to earlier levels to see if I can tear through the level faster or in a new way, but if there were also hidden treasures that could only be gained by replaying levels with ghosts you didn't have yet, that would be awesome (like not unlocking an "Earth" ghost until level four, but then if you take them back into level two and do a tremor then it topples a garden statue and frees a hidden ghost or additional plasma you couldn't get to earlier--that sort of thing).