For me it's mainly about humour. Ms. Marvel, Superior Foes, Hawkeye, Captain Marvel, (Waid's) Daredevil, Spidey, those are my kinda characters because they're cheerful to spend time with. I'm less inclined to read series with mopey protagonists; if I wanted those I'd find my old Final Fantasy VIII game!
Out of those Hawkeye and Daredevil can be kinda downers sometimes, but things like the crisp cartoony art or delightfully zany plots can still keep the tone overall pretty light.
I'm not sure beyond that (this is why I'm a reader, not a writer :P ), but perhaps something slightly unique? I wonder if it could be done to write a comic in which the main character speaks no or very little English, perhaps learning over time? Should be doable since visual language allows to express so much; plus it could lead to hilarious situations. (I'm thinking a date with Johnny Storm who is forced to communicate in gestures and fails miserably.) Not sure about a power, maybe just a regular Marvel U citizen who just happens to stumble into a bunch of contrived situations. Or they could just produce a lot of foam, something fairly low-powered but visually interesting.
It'd also be refreshing if bad guys had some zanier plots again. Not always out for the main character, but just idiots trying to rob the local sandwhich store. Our hero(ine) stops them but OH NO they were actually going to give the sandwhiches to elders in a retirement home or something. Does our hero(ine) serve them a knuckle sandwhich or will they help the robbers to blenderise a few sandwhiches for a toothless elder? (WHY NOT BOTH?!)
Or how about a therapeutic bowling alley visit with some other superheroes who have a lot to get off their chest? Spidey could use someone to listen to him without judging (plus our main character can't really reply much), although maybe the bowling ball would get stuck to his fingers...
Anyway like you can see, these are scenes at best, not enough to fill a series and I doubt Marvel will suddenly call me to write this for them.
(But hey I'm available?)