I don't think a T-Bolts movie would need to keep the group's secret identity as the Masters of Evil a secret to the audience. To me, the main appeal of the group is not the twist (partly because I'm reading and loving this run over twenty years after it originally came out), but the gradual transformation of these villains into people who genuinely try to be good and appreciate what super heroes try to do*. I find it so interesting, that all it took for half the Masters of Evil to become good citizens was a stable environment and not being hated by the public. And then they have an ideological over following Zemo out of loyalty, or try to keep being good despite worse circumstances. That's the main two thing I'd want to see in a Thunderbolts movie. Although the whole concept would be better suited for a tv show.
*Side note, and I am literally only using this as an excuse to talk about comics: One of my favorite early appearances of them was in an issue of Spider-Man Team-Up, where Spider-Man is framed for some random bullshit, and the Thunderbolts, at the time in good standing with the government, get sent to capture him. This amuses Mach-1, formerly long-time Spider-Man foe Beetle, who obviously sees the irony in this situation, and remarks that he's probably one of the few people who are 100% sure Spidey's innocent. The Thunderbolts and Spidey of course end up teaming up to defeat the baddies and Spider-Man ends up saving Mach-1's life, which just baffles Mach-1 who then ends up giving Spider-Man the information needed to clear his name entirely voluntarily. Like, he didn't need to do that, but he did anyway, because it was the decent thing to do. Seriously, villains becoming heroes is probably my favorite comic story type there is.