Elsewhere I saw someone point out that megas that change type almost all have moves of the new type in their level-up learnset. Chikorita gets disarming voice, barbaracle has brick break and close combat, feraligatr gets breaking swipe and outrage, and clefable learns air slash. Ampharos also learns breaking swipe by level up, Charizard gets a few dragon moves, in ORAS Sceptile got dual chop. Altaria and Audino get fairy moves. Mewtwo gets aura sphere. This is obviously a good idea from a game design perspective, since it's kind of annoying for a mon to gain a new type but not naturally learn any attacking moves of that type.
A notable exception is Pinsir, which does not get any flying moves by level up, but this may be because in XY it had aerialate to turn normal moves (of which it had vice grip, thrash, and double hit) into flying type attacks. They didn't add any flying moves in ZA to compensate for lack of abilities, though. I guess they figured no one would care to use poor Pinsir.
So, for the mons already in game, if we assume that GF won't add any moves to learnsets when the DLC drops, and that they won't consider the moves Alphas come with, then we can scout for possible type changes by looking at the level up moves.
Absol gets shadow claw and Air slash in ZA (both newly added from SwSh), and also has future sight (previously in its SwSh learnset) and of course normal moves in Slash and quick attack. So we could get dark/flying, dark/ghost, or even dark replaced by ghost for pure ghost or even ghost/flying. Or, possibly even ghost/normal: if any mon were to gain Normal type as a mega, a ghost/normal mon seems like a good fit. It meshes well with Absol's yin yang theme, and I can already see the Champions trailer showing an absol mega evolving into mega absol Z to avoid a fighting move. While a dark into ghost/normal would feel a bit like it's copying off of zoroark's homework, I don't think that would deter GF.
Garchomp doesn't really get any additions. It has crunch and slash. So could very well just be another dragon/ground, maybe more specially oriented. Or maybe it pulls an Aggron and loses a type, becoming pure dragon, which would be a first -- I think all other dragon megas are dual-typed. Pure ground is also possible of course, but I think the idea of removing the dragon type from something would give GF's management an aneurysm.
In going from SV to ZA, Lucario has bone rush swapped out for bulldoze, and learns dragon pulse and extremespeed in both games. So, no clear indicator of a potential type change, but I wouldn't be surprised if it went dragon with an eastern dragon look, or even fighting/ground and leaning into the anubis inspiration some more. (I doubt it would lose fighting instead of steel).
Finally, both Meowstic get disarming voice as an attack, while female also gets magical leaf and dark pulse. If this is a pyroar situation despite the different learnsets, then the only dual-type option seems to be psychic/fairy. But Meowstic also seems like a good candidate for just keeping its pure psychic typing.
Now of course, GF can always throw curveballs. They could update learnsets to add moves despite only doing that to fix errors in SV. Maybe some mons will get an -ate ability in Champions and they don't bother adding moves of the changed type to the learnset, just like Pinsir. Or maybe there are shenanigans with new moves learned over level 100, such that the moves stick even if the level is temporary. Or maybe GF just says, "it learns a <whatever> type move by tm, good enough."
