I really hope that the big bad for Season 5 is The Fixer. It ties things back into HYDRA and much like the appearances of Blizzard and Mister Hyde, Fixer will help set up Thunderbolts.
When you look at where the other potential T-Bolts can come from, they all have good places to be introduced. We already have Zemo, Hawkeye, Leader and Shocker in the films. Luke Cage, Elektra, Punisher, Nuke and Ghost Rider are already characters the TV series. So it isn't outside the realm of possibility. The only T-Bolt who outright died is Crossbones. And Paladin is rumored for Jessica Jones Season 2.
Beetle, Joystick and Scorpion - Spider-Man: Homecoming 2
Red Hulk and Skaar - Incredible Hulk 2 (although I'd rather wait for 3 for RH and 4 for Skaar)
Moonstone - Captain Marvel
Atlas - Luke Cage Season 2
Ghost - Iron Man 4
Bullseye - Daredevil Season 3
US Agent - Captain America 4
Radioactive Man - Ant-Man and The Wasp
Songbird - Jessica Jones Season 3 (I know she's a Captain America villain)
Ragnarok - Thor 4
Man-Thing - hypothetical Blade TV show
Yelena Belova - Hypothetical Black Widow film
Taskmaster - hypothetical Shang-Chi TV show
Deadpool, Juggernaut, Gorgon and Daken are owned by Fox but they're the only exceptions. Everyone else is somebody who Marvel can either use outright or somebody who they'll need permission from Sony to utilize but they have a really good potential roster and most of these characters can easily be introduced in Phase 4 although Atlas, Fixer, Bullseye, Beetle, Joystick and Scorpion can all be introduced at the tail end of Phase 3.
And even when SHIELD is over, there's room for the cast to move on to other roles within the MCU. I feel like the best place for May, Quake, Mockingbird and Hunter is a Shang-Chi Netflix series. Coulson can move on to Iron Fist where he'll be informing Danny of a larger threat than The Hand Namely HYDRA for Defenders 2 which should adapt Seven Capital Cities of Heaven. Mack, Fitz and Simmons should appear wherever it's convenient. Ghost Rider needs to get his own show.