8. Galactus - Bryan Cranston
One of the hurdles facing the MCU in Phase 4 is the question of how to top Thanos, a villain who has literally wiped out half the universe. One such answer lies in Galactus, a cosmic entity who devours worlds merely to sustain his existence, such is his power.
The Devourer of Worlds likely wouldn't be in the first Fantastic Four film, but since Marvel will almost certainly make a franchise he could be seeded for an arrival in the sequel, and even built up into the big bad for a team-up event, especially as Phase 4 is focusing more on the cosmic branch of the MCU.
The main requirement is for a booming voice, since the character would be CGI/mo-cap, but it's not just volume: you want the voice actor to really be credible as a villain, and to convey all the menace of one of the most powerful evil-doers in the galaxy.
Enter Bryan Cranston, who can be completely threatening when he turns it on (see: his descent into Heisenberg), but would be able to bring plenty of weight and complexity too. He's flirted with performance capture before, albeit in Power Rangers, but with Marvel's team he should be capable of something more in line with what we've seen from Josh Brolin, wherein his own performance isn't lost, but simply enhanced.