I would vote for option 2. Removing a few levels seems like less of a loss than the option 1 of removing a few worlds. You can always decide to revisit option 4 at a later date.
Option 3 seems like it could be more replayable, because the single level in each world is randomized, but at the same time, could be more stressful to always have a large boss chasing you, and it wouldn’t be possible to “learn” the level or practice in a more peaceful setting to get used to the game. Plus, one has to hope the automatic level generator is able to make levels as fun as the ones you would custom make. Keeping rainbow world seems like another plus of this option - no need to wait for a DLC or free update later.