First, here's some potential 7800 ports that I might make:
-Thunder Force (Sharp X1/Fujitsu FM7)
-Bad Dudes vs. Dragon Ninja
-Space Gun (Will have Light gun, Mouse, and Trak-Ball support.)
-Marble Madness (Will have Trak-Ball support.)
-The Fairyland Story
-The Legend of Kage (Possible dual joystick support.)
-The New Zealand Story
-Chase H. Q.
-Gradius 1 & 2
-Turrican 1 & 2
-Lemmings (Will have mouse support.)
-Osman/Cannon Dancer (with spelling and grammatical fixes.)
-The Cliffhanger: Edward Randy (with improved controls, I might add.)
Before I go, here's Dark Edge, a sprite-based 3D fighting game from Sega:
I hope this gives someone here an idea or two...
Update: I also wanted to say that many of these ports can also be done the other systems mentioned.
Update 2: Come to think of it, Quadtari, Comlynx, and Team Tap support are also on the plate.