I'm not a legal expert, so God knows I won't be offering any legal advice.
That said, the appearance angle pertains to trademarks rather than copyrights. Copyrights protect specific productions--a book, a single issue of a comic--and lasts for a limited period of time (supposedly; more on that below). Trademarks protect the "look" of a product--a costume, a trade name, a logo--and seems pretty much to last forever so long as its owner actively protects it from infringement. The main test seems to be: would a person off the street, seeing your creation, mistake it for the trademarked one? If they see your superheroine, might they think she is Wonder Woman?
The only reason I know anything at all about this is because in the late 80's I pencilled some (unpublished) comics for a Mattel project. The company provided extensive model sheets and warned us to follow them carefully. It was explained to me that in He-Man's heyday the company almost lost the character to the Public Domain in an infringement lawsuit. The other side argued that everywhere he appeared (TV, boxtops, storybooks, comics, ads) He-Man looked different. He didn't have an identifiable look that could be trademarked. They almost won, but of course money trumped all and Mattel prevailed.
The hijacking and near-total destruction of copyright law by big media corporations during the last few decades has made for some very treacherous shoals. In the name of "protecting the artists" they've engineered near-eternal copyright over everything they feel like owning. Artists be damned; they just want to make sure money flows from artistic endeavors to the corporation and the corporation only, forever. That's why I worry that creators are asking for trouble if they try to make something from characters with even a remote chance of being claimed by The Majors. If you look like you might be making money they'll come calling with their cease-and-desists. If you fight, make no mistake, they'll win. Their money has bought the laws and elected the judges. But I've done this rant before, so I'll stop here.