Honestly, since Ridley appears to be just about out of the running (fingers crossed that the theorists are right but not betting on it) and Dark Samus being an Assist Trophy I honestly really only see Adam as the last remaining possibility for another Metroid representative. He's not exactly the popular choice and he's pretty recently been introduced (at least as a physical being) into the series, but Rosalina and Greninja have shown that both of those factors aren't that much of a stumbling block.
On the flipside, he's really important to both of the Metroid games he features in, CAN physically reappear in a new Metroid game so long as it takes place before Other M (and considering how the series tends to jump around its timeline, it's not too unlikely), or even can stick around as a CPU for post-Other M games. He could use Other M's weapons like the Plasma Cannon and the Ice Pistols, as well as some of Samus's unused abilities as well. Alternatively, in the unfortunate event that Snake gets cut, Adam could easily inherit his moveset, albeit with some slight alterations. Yeah, Anthony is more popular, but that "popularity" is pretty tiny, and is almost entirely due to his meme. Adam has seniority both in importance to the series and Other M.
I don't see it as too likely, but I see him as the only remaining character that would've been considered for Metroid. I honestly thought the guy was an interesting character, just that the game's mechanics for unlocking powerups made him seem like more of a jerk than needed, so I really wouldn't mind him. Ridley would obviously be the ideal rep, but in his dire straits, it's best to cover all bases IMO.