Pieman0920
Smash Master
Nothing at all. Outside ofHey Pieman, did he say anything about where Poseidon comes in?
Hades
Dedede hasn't been a final boss since his debut, barring spin-offs. Doesn't make Dedede less important, and it should do that for Medusa.
Dedede had many games though, not just two. While she probably has a higher chance than Hades or Viridi here, the tie between her and Palutena is sort of broken if she doesn't show up later, since Palutena evidently doesn't lose her role as the game goes on.