I'd say that the appearance of new unannounced galarian forms is on the line of "Possible, but unlikely".
Since this is not a full release but rather a DLC, it's a bit weirder than previous times.
I'd usually be inclined to agree with that reasoning, but let's consider a few things -- how much do we truly
know about the features in Isle of Armour, even the ones we've technically been told about?
We've got a way to get more Apricorn Balls easily? What's the method; how many a day would we get; are there any limitations?
We're getting Move Tutors, including new moves? Have we gotten anything even resembling a small list of which moves would be included so far, beyond moves like Grassy Slide that were barely alluded to?
More Trainer Customisation options? We were shown a lot in the initial trailer to be sure, but since then we've been given no details on how to get them or which ones will be in which pass, beyond the details on special pre-order and event exclusive ones.
Max Soup, the Cram-O-Matic, the Exp. Charm? Granted a lot of that is self-explanatory, but there's still been no elaboration on how soon you can get them, or what sorts of things you can even get from the Cram-O-Matic to begin with.
Returning Pokémon? Granted, this is one that we've gotten a
lot more information on -- we were given a very solid quantity of around 100 creatures being added back, and we have seen quite a large number of them already. Still, even though half is a
lot... you'd maybe expect almost all of them to be shown off given the nature of DLC, right? Like, maybe 90 or so and just leave out some Legends? Consider just how comprehensive Smash Directs are, for instance; which seem very intent in going into every single little pixel in detail for 5 minutes at a time. SwSh's Expansion Pass pre-release has just... not resembled Nintendo's usual DLC pre-release materials at all.
And that's before getting into any other features we don't know about, or exploration of the island beyond a few glimpses here or there; heck, we didn't even get told what Slowbro's new evolution item is! While I'm very confident in saying we know more about Isle of Armour pre-release than we did base game SwSh, I definitely feel like they're sticking to their new tactic of giving us as little information as possible. As such, while I don't think we'll see many more - I will be very surprised if we see more than two families aside from Slowpoke - I definitely think we'll see new Galarian Forms come Wednesday. There's a lot of ground still not covered by pre-release and that's made me pretty excited going into this!