Given what we know of Nathan Fillion's role so far in Guardians of the Galaxy Vol. 2, it's easy to say that his appearance on various posters as Simon Williams, also known as Wonder Man, is nothing more than a cameo for now. The assumption is that he'd be appearing, at earliest, for Infinity War. That's not totally unlikely. Wonder Man has been a long-time Avenger, and so far, we've only seen a few posters on a theater on Earth. For all we know, it could even be a flashback. But even if that is the case, that may not be all we see of Mr. Williams in the film, cause he wasn't just an Avenger. He was also a Guardian of the Galaxy.
Don't see him at first? Take a look at the guy in green. That's Simon Williams, also known as Hollywood, as part of the Earth-691 Galactic Guardians. He survived for centuries in space, and became a galactic movie star-something that could fit him in whether the theater scene is a flashback or not.
Now, in the comics, Wonder Man was created by Baron Zemo and the Masters of Evil to battle the Avengers. But an interstellar event happening on top of him, say, at a film festival, could definitely provide that ionic energy even if he doesn't join the Guardians in this film. Given that the character is portrayed by Nathan Fillion, it seems unlikely that he'll stay as just a cameo for long.