There's definitely a possibility of doing something cool with DK Jr. And after going through all this effort to create DK Remix, getting the hardware designed and built for it, etc... the barrier for doing a project for another game that runs on similar arcade hardware is much reduced. The "Remix" hardware could be used for a DKJr Remix project idea, or even maybe for other Z80 games as well.
It would still be a big project and it would take time, but if I put my mind to it there could be a Jr Remix by the end of 2016.
Or if others had ideas or wanted to try their hand at a project, we now already have a board to make it possible.
(We can't change the graphics, or add new sounds - beyond reusing existing sounds and graphics in new ways anyway. But the main program code / game logic can now easily be replaced. And the board will allow more program code for bigger games, more levels, etc.)
If someone wants to start the ball rolling, the first thing we need is a disassembly of DK Jr's Z80 code. From there we could make new source code that would re-assemble into working game code which could then be modified and expanded upon.