Ohhhhh Punch-Out! I gets it , I gets it.
Having never tried sprite art, I can't think of much to say, haha, but yeah... maybe it might be interesting to use some coloured lines to outline Mac? Like, instead of using black lines to outline everything (like you've done), you could try using dark green outlines to outline his shorts, dark purple to outline his shirt, dark red to outline the gloves, darker brown (or maybe pink) to outline his skin... I dunno, I'm just thinking back to fighting game-type sprites (like Street Fighter II or Marvel vs. Capcom), and those sprite artists often use coloured lines instead of black lines for some reason. A lot of animated cartoons (like Disney films and such) also use coloured lines to draw their characters.
Examples:
T.Hawk from Street Fighter (they use blue lines on his headfeather, brown lines on his boot, blue lines on his jacket, dark brown outlines on his skin....I think the only black lines in the entire piece might be his eyebrows):
And here's a random scene from a Disney movie (first one that popped up in a Google search for "Disney Movie"). The artists used blue lines on Jasmine's clothing, brown lines for the character's skin, dark yellow lines for Aladdin's clothing...and again black lines on their eyes to help "pop" their eyes out and draw your attention to their eyes):
I'm not 100% sure why cartoonists use coloured lines, but I assume they have a reason (which is why it's so common), heh. And maybe doing that to your picture of Mac would be a bad idea (because maybe Mac actually had black outlines on the NES so it would be more appropriate to just leave him with black lines), but I dunno, random thoughts, random thoughts, haha. If you do ever use coloured lines, though, I would keep the eyes/facial features dark black though, so that the eyes 'pop out' (because of the contrast) clearly and are very easy to read.
Man - I'm rambling. You probably weren't expecting a ridiculously long post like this, haha. It's probably completely irrelevant too. I'll stop now!