I agree that Samus/ZSS, Ridley and Dark Samus are first, but is it REALLY that much of a stretch to say that Adam's next in line?
Mother Brain and Kraid has consistently been too large, Anthony and M-B are, as mentioned earlier, "flavors of the month" and the MPH/MP3 hunters are generic, unimportant and uninteresting.
Adam, as I've said before, had big roles in Other M and Fusion, and I know that he's dead at the end of Other M, but he's actually got a pretty decent chance of returning in more games. He's been established as pretty important, and considering the Metroid series's tendency to jump around in its timeline, Adam could easily appear in any game that takes place before Other M, considering that he wouldn't be dead at that time. On the other hand, if a new Metroid game ends up taking place after Fusion, Adam would be her computer again. Realistically, the only way Adam has no chance to show up in a Metroid game would be if the game takes place between Other M and Fusion.
However, I do admit that Higgs also has similar chances to show up again. However, I don't feel that he had nearly the amount of impact that Adam had, nor do I think Nintendo would be as eager to bring him back as they would Adam.