Probably one of the most interesting new races in the Spelljammer Astral Adventurer’s Guide is the plasmoid. Plasmoids are oozes, not humanoids like many other races in the game. This is an important distinction, as it makes them immune to spells like hold person, which can only target humanoids.
Given the way that Ability Score Improvements are handled in the Astral Adventurers Guide, it’s not possible to say what classes they are best suited to based upon the bonuses they get. They do have other traits which suggest what sort of activities they might excel at.
Plasmoids are amorphous, and so can squeeze through spaces as small as an inch wide as long they are not carrying anything. Seems like a neat trick for a plasmoid rogue to be able to use. It also would also increase the tension of having the rogue split from the party to move ahead and scout. Once they slough their armor off to do a bit more exploration, they are alone and naked.
The Shape Self trait allows the plasmoid to use an action to change from a blob to a humanoid for as an action. Secondly, with a bonus action, they can extend a pseudopod up to 10 feet. Picking up tiny objects and opening doors is all this light-duty appendage is good for. This also seems like a good fit for a rogue. Plasmoids can also hold their breaths for an hour, giving them a bit of an edge in wildspace and underwater scenarios.
One last interesting ability to note is that they have advantage on ability checks made to grapple or escape from grapple. I really wish they’d given the plasmoid the full grappler feat. This would make it able to attempt to pin a target it had grappled. That would make a really strong argument for creating a plasmoid character with a really high Strength score.
We have yet to see a plasmoid enter the arena as a contestant. I think there may be a bonus for the first players to bring a goopy friend to the fight next season.