Charles "Trip" Tucker
Charles Tucker, known more regularly as Trip, was easily the most popular Star Trek: Enterprise character. He was fun, jovial, interesting, and charming. He was a salt of the Earth type, but he wasn't some bumkin who didn't know what he was talking about. His death at the end of the series remains a top terrible moment in the history of the franchise and bringing him back into the fold in some way shape or form could help repair some of the hurt that the Enterprise series finale caused. Personally, I think a film that centers on his 'return' would be huge for the Star Trek fandom.
Jonathan Archer
While it may not have been in the way everyone has imagined, but every captain from the Original Series to Voyager has returned to Star Trek in the last few years. Jean-Luc Picard and Kathryne Janeway came back in Star Trek's Picard and Prodigy respectfully, while William Shatner's version of James Kirk came back in a Star Trek-affiliated short film. Benjamin Sisko also came back recently but in a comic book. That comic is canon, but we're unsure if the short film that Shatner oversaw is. Though it's implied to be. That means that the only captain from that era of Star Trek that hasn't returned is Jonathan Archer. With Scott Bakula still acting, it's high time that Star Trek brought back Archer one more time.