Fan Fiction Question

CONCEPT

If time permits, and I'm not sure it will, I will be writing some fan fiction this summer.
My two major projects are Harry Potter and a framework for Madeleine L'Engle's works. In the case of the latter, I've gotten caught up in trying to make it through the canon, plus find our her particular stance on fan fiction.

However, I'm thinking of branching out in a somewhat benign way, and I'm interested in opinions here.
I'm considering using some of those grand old original NES games for source material, and I've got it specifically narrowed down to:

CASTLEVANIA

MEGAMAN

NINJA GAIDEN

The question is where to start (time permitting)?
So I am requesting opinions.
This isn't a vote as much as an open-ended question. Where should I put my time and energy, and why?
Please let me know if you've got any ideas...

~ Connor