Firsly, +1 for the formatting. It makes the post so much easier to read.
I've been thinking of which non-PHB subclasses could get added. For the purposes of this discussion I'm going to try to list 1 subclass for each PHB class based on likelihood (speculative of course) and my personal preference. I'm also going to try to skew toward more popular/established subclasses so I would be looking at more Xanathar's and Sword Coast Adventure Guide than Tasha's or Fizban's.
By and large, I think this is not a game I'd say I'm particularly interested in. But if I had to play it, or design it, I'd have gravited toward slightly different criteria.
Regarding standardness, I feel that Tasha's Cauldron Of Everything (TCOE) and Xanathar's Guide To Everythin (XGTE) are much more "the standard expansions" than the Sword Coast Adventurers' Guide (SCAG).
Regarding likelihood, I think everybody's guess is as good as a shot in the dark. But I'd like to consider the factor of "amount of work necessary to add this Subclass to the game". If a Subclass has purely numerical-mechanical abilities (like the Eloquence Bard), then it feels pretty easy to implement. If a Subclass requires additional visual effects (like the Beast Barbarian or the Wildfire Druid), then it requires a bit more work. Visual assets (typically, animation) need to be created. I'm assuming modders will add all the Subclasses and Classes (and do so way before a potential, putative Larian expansion), and I'm assuming modders can more easily add the numerics and mechanics. So I'd rather have Larian provide the visual assets.
Regarding the story-fit of the Subclasses, I think the 3 psionic-flavoured Subclasses are a must-mention. For these Subclasses, one could imagine the characters developing these special/aberrant/psionic abilities as a result of the implanted tadpole. Those are
- Aberrant Mind Sorcerer,
- Soulknife Rogue,
- Psi Warrior Fighter.
Finally, I would consider how well, or poorly, Subclasses would work with the existing UI or game mechanics. The Shepherd Druid, just like the Necromancer Wizard, are geared for a summoner gameplay. Much as I love the idea of a Shepherd Druid, I don't think that playing a summoner will be particularly enjoyable and practical, with Larian's chain control scheme for the party. Although, of course, one may argue that the classicness and standardness of this gameplay style in RPGs is a reason to revise the party control scheme, instead of calling for not implementing this gameplay style (much in the way that the poorness of the party control scheme is not good reason to advocate against a party of 6, but instead large demand for party of 6 is a reason to revise the party control scheme).
