You can easily sell Doom as the next overarching big bad. That's what makes him great -- Doom is a human, and proves to be the most dangerous villain ever. If it's just Kang or Annihilus, it'll feel like more of the same as another cosmic villain. Doom, on the other hand, will provide a new perspective in terms of being a villain, that'll contrast him nicely to Thanos.
Think of it: Thanos is an alien from a distant world who thinks little, if anything, of Earth, and believes it's his burden to balance out the entire universe. Doctor Doom is a human from our world who believes Earth must be protected from any threat, and that he's the only one who can do it. That's what you call a "contrasting sequel antagonist".