Victor Doom is probably the best villain in comic book history, and I think he should be the main villain of MCU phase 5 here's why. First off his origin cloud tie into Avengers: Age of Ultron, instead of him being from Latveria he could be from Sokovia (both are in Europe) and he and his mother could have been forgotten while S.H.I.E.L.D & the Avengers where getting everybody off. His mother could have died. This can lead him to hating the Avengers & all other superheros, his armor could also be made from Ultron bots. Second in the Comics he the ruler of Latveria (maybe Sokovia in the MCU) and he could have been one of the 117 country leaders to sign the Sokovia accords (or even one the writers sense it is called the Sokovia accords). Third he could have been upset when the Avengers stared fighting again in Infinity War & Endgame. that's all the stuff I have about him joining the MCU, what do you think?