Here would be my MCU phase 4 Thunderbolts treatment
"Thunderbolt" Ross has had a few years of enforcing the "Sokovia Accords" managing the Raft, dealing with assorted enhanced ppl and Inhumans etc
The registered Avengers are too public and Cap's secret avengers are out of his control. Ross confides in friend and associate Norman Osborn (likely already introduced in Spiderman). Complains about Shield and the Avengers and his lack of control over domestic Terran situations when alien treats still loom. Osborn gives him the idea of the Thunderbolt Initiative; an off books project of appropriating enemy tech and personal to preform black ops. A secret, parallel force under his strict control. Even recruiting from inmates of the Raft to hunt down future prisoners or "Thunderbolts"
Even though it is called the Thunderbolts and Ross is a catalyst he wouldnt be in the picture much. he would hand much over to Osborn and Osborn would in turn form the initial line up, tainted by his own agenda. (it would actually be best for Ross to just brood over the Avengers and Thunderbolts and get frustrated with Osborn's secrecy until Osborn goes all Green Goblin, kills Ross, erases all record of the Thunderbolts and beings using them unabashedly for his own means. but that would be more like end of act 3 or beginning of movie 2)
step one for Osborn under Ross's direction is securing the prison for the enhanced. The raft is highly inadequate. Time for a prison that can hold ANYONE (or anything) that NOTHING can escape from. enter The Negative Zone. a great way to introduce the concept to the MCU.
Dr. Chen Lu has recently been transformed into the Atomic Man or the Radioactive Man etc. He is being imprisoned/quarantined in china. One of the last projects he was working on (and has been allowed to continue to work on in captivity) is cross dimensional travel into the Negative Zone. He is recruited to help them form a secure gateway to the Negative Zone and design them a prison. he asks why he should help them create an even greater prison for himself and Osborn assures him he wont be a prisoner, if anything he will be a warden, or better "King of the Negative Zone."
Meanwhile to go along with the heavy, Negative Zone, sneaky villain intentions we need some on-the-ground relatable characters. The way to get Baron Zemo into the movie and to develop him as a character that in some way forms his comic counterpart is that he knows things about Hydra assets and has shown the ability to be brutal in his tactics. He may also claim to have uncovered information connected to the 10 Rings etc. so he is recruited and outfitted to be the lead on a number of missions to acquire Hydra assets, neutralize hydra targets and run counter-ops against even the Avengers and Shield etc.
for his team you can introduce Moonstone or Songbird. Depending on their status from Phase 3 you can use Hawkeye or Scarlet Witch (in the same way as she took on some of sue storm's roles in Civil War) Baron Zemo should get some sort of a power upgrade over the course of the movie like a version of super serum or something that improves his mental capacity to be even smarter, maybe even have limited mental superpowers. This will give him more legs for MCU future. you can also fuck up his face and give him a reason to sport a hood in future appearances. (and someone to hate more than Cap America)
OsCorp could have purchased the remnants of Hammer Industries to expand their hardware manufacturing part of their biotech dominated portfolio, giving Osborn a connection to Justin Hammer. Hammer is transferred from seagate to the Raft and recruited to be like a anti-Q branch for the team. so some Sam Rockwell action can be thrown into the mix with him providing some Ironman tech rip offs as well as some humor and reluctant participation in their quarrels.
other very expendable Thunderbolts id like to see would be Doctor Octopus. It would be too reduntant for him to appear as a primary villain in the Spiderman movies. so introducing him as someone connected to Osborn's new interest in weapons and robotics etc who he hired (perhaps from prison like others) gets a version of his Octopus suit and killed off in act 2. Also i would love to see the Hob Goblin or Jackolantern. I hope the MCU treatment of Osborn is that he has the Green Hoblin inside of him but keeps it contained. never jumps on a hoverboard and only ever gets in a couple brief fist fists to tease at his hidden power. and again a hoverboard gliding goblin in a Spiderman movie would be repetitive. But a criminal Osborn enhances with some of his own medicine and Hammer tech etc who wears a hood and acts like an idiot throwing pumpkin grenades around until he gets himself killed would be great. (anyone whos read the Punisher storyline in Civil war knows how satisfying it is to see them go down)
I would also use the opportunity to give closure to the Blonsky storyline. Ross would pressure Osborn to use him much like he wanted him as an Avenger over the Hulk. Either use him as a second act action scene as he fucks shit up while being prisoner transferred to the Negative Zone Ultramax (mcu version of 42, whatever they end up calling it) or use him as someone intended to be an OP member of the Team who Ross wants strict measures to insure he can be controlled that end up killing him. He could introduce the Suicide Squad style measures in place to control the criminal recruits. Something like a weaponization implant of the cross-dimensional travel that makes an unstable pin hole that implodes the victim through a pin-hole-black-hole only to scatter the bits into the Negative Zone.
(Other members of the line-up could obviously be Claw, Venom, Ragnerok etc. depending on their use at the time in the MCU. Even Elekra or The Punisher depending on their status with Netflix at the time
In the end the superficial mission (blow up or kill Hydra X/ Steal Y etc) is accomplished and the Thunderbolts remain secret but more importantly Osborns agendas advances, Zemo is stronger and has freedom within his partnership with Osborn to make moves on the Avengers etc, Some loose ends in the MCU can be cleaned up in a fashion as close to an R rating as possible. Probably a couple members of the squad go rogue and a couple foes from the movie join the line-up. In the flashback scenes about how recruits originally got captured there can be potential for Avengers cross overs (spiderman webbing up doc-oc) as well as scenes of actual Thunderbolt vs Avenger content.
most importantly some villains would be developed. everyone knows the MCU has a problem with villains that sustain over the films and villains with depth to their motivations. Not only could Thunderbolts develop some villains in accessible ways but it could be the brand under which most of the Masters of Evil content is fleshed out. Ross giving Osborn free reign over powers which would otherwise be kept to individual crazy villains, the Avengers and Shield could lead to some reason and resources behind the Masters of Evil.
also depending on the line up of MCU phase 4, a bad-guy focused movie could flesh out the Inhumans, Atlantis and more of Wakanda. (even Kree/Skrull) just like how the other movies have expositional drops about wakanda etc. most of the time we learn the deep exposition its from Starks or Shield's or even Nova Corps databases. OsCorp and other villains must have their own wealth of info on the playfield. the cynical and exploitative nature of Osborn would allow some scenes about plotting against/ stealing or even recruiting from Inhumans or Atlantians. It might be hard for them to accomplish those areas as stand alone movies (competing with public perceptions of xmen and aquaman etc) so side projects like Thunderbolts have potential to mine that content.