The following images are just mockups, this romhack combines existent romhacks, SMB2J and SMBDX (GBC):
Separated palettes inspired by SMB Beginner hack: https://www.romhacking.net/hacks/2012/
Also Boo from SMBDX (GBC) is introduced here:
You can see the world maps like SMBDX (GBC)
You can play two players as this Romhack does: https://www.romhacking.net/hacks/4180/
Left-Right Scrolling to get back any time in the level.
Toad and Peach animations like SMBDX (GBC)
This romhack is possible, sadly is hard to make and I don't have any knowledge.