changing the frame rate on Ghosts 'n Goblins for the NES?
Not easy. I'm not sure about what frame rate the game actually runs at, but improving it would be a hell of a task. It's not like 3D games, games on the NES were just designed to be like that, so changing that design would be more trouble than it's worth. Just play the arcade version.
add Super NES Mouse support to Nintendo's SimCity?
Now THIS is more doable. One could analyse how mouse input works on Mario Paint and the few other games designed to use the mouse, then try to replicate that in Sim City. If there's a game out there that supports both controller or mouse, even better. The tricky part is telling the game that that's what you want to do, but I really don't think it's outside the bounds of possibility.
Given how much people like this version over all others, I'm surprised nobody has tried to do it. I would, but alas, no time.