Here's an attempt at making a mock up of sorts, featuring "Mikey" aka the character you see there. My pixel art and tiling skills still have a long way to go but I see this as a good start.

I tried making this in-line with the limitations of the Sega Mega Drive and while I think I didn't take advantage of the whole color palette capabilities, this is but a start.

The first drawing is actually the second iteration I made trying to improve on the first one, with more tiles than the second drawing, which is a bit crude but okay. The "health bar" is alright, although I made it off-centered in the second iteration, but there are many elements I plan to add to the UI in future mockups.