The CW's Flash is centered on alternate Earths, but could one such reality contain a version of Grant Gustin that's a Marvel superhero?
In all seriousness, it's likely that Gustin will never appear in the Marvel Cinematic Universe as a superhero as he's currently busy playing Barry Allen on the Arrow spinoff. Gustin is well aware of this too, as evidenced by his answer to a fan question at ACE Comic-Con about the MCU hero he'd like to play.
"Spider-Man. I love Tom Holland, but Spider-Man. I love Andrew Garfield too, while we’re talking about it."
Tom Holland will be playing Sony and Marvel's Spider-Man for the next decade or more so the chances of Gustin starring as the wall-crawler are slim to none. Still, there have certainly been more than a few fan suggestions in the past that Gustin would make an excellent friendly, neighborhood Spider-Man.