I also have tried to mugenize some non-fighting game characters now and before. I don't know how to draw 2D sprites at all.
For any lacking sprites I try to use Franken-spriting way to supplement.
A good Franken-spriting web site here: http://scrollboss.illmosis.net/tutorial_frankenspriting.php
"Franken Spriting 101" YouTube video by MFFA boss Ryon here: https://www.youtube.com/playlist?list=PLIwEWBFlq_nqdRXtIA9rccQXx29eSHPlY
"Mugen Character Tutorial(s)" YouTube video by MFFA boss Ryon here: https://www.youtube.com/watch?v=79fDiVA9ZiE
MvsC-styled characters are not easy to mugenize (most MUGEN Pro's said). I have 2+ years of MUGEN character making experience yet I still haven't tried any MvsC-styled mugenization because I know it is not easy to me (because I don't know much about MvsC).
After you have gone through those tutorials, try to improve your Nebula and Gamora. And, before trying some MvsC-styled mugenization, I suggest you to try to study some other MvsC MUGEN authors' works to see.....
hint: http://mugenmultiverse.fanbb.net/f17-mmv-awards-discussion-area
Good luck in your MUGEN'ing!