Thunderbolts* secret mission to potentially rescue...
Dr. Doom. So yes, go ahead and laugh at this theory but just fun speculation that if perhaps the real title of Thunderbolts* turns into "Dark Avengers". then perhaps one of their missions is to rescue Dr. Doom.
Taken from the Dark Avengers Wiki (deleted some text, but to piece this together for you) "The initial line-up consists of former Thunderbolts members and new recruits, including the Sentry. The team goes to Latveria to rescue Doctor Doom from Morgan Le Fay."
Now I'm not saying this is the whole point of the Thunderbolts* movie, but perhaps if there is a "twist" to this movie is that a final mission or maybe post credits mission is to free Dr. Doom.
This movie also comes out just a few months before F4 First Steps. I don't see why it couldn't help grease the skids to get Doom introduced into the MCU which would also allow for a lot of fan speculation for 2 months before F4's release.
Just fun to think about, but knowing that some of these Thunderbolts* characters will be returning for Doomsday\Secret Wars then it seems smart for Marvel to have a few direct breadcrumbs as to why they are a part of those bigger films?