Well, they have a lot of creative opportunity if they're pushing this "alternate story" narrative with these games. One of the posts I saw somewhere speculated that this could be something like, "oh, instead of coming to Alola in 2016 or so, it's like the protagonist(s) never showed up and moved in a year later." I saw a video with some similar speculation that it's like they move there at about the same time, but start their actual adventure maybe a year later, since the family looks more settled in the short trailer we have so far.
I think that gives them a ton of time to work with new forms. They could introduce new Alola forms and say that they had been discovered recently, or that new ones continued to appear throughout time. You could even bring in new Ultra Beasts with the emphasis on "ultra," since (as ShadowForce720 stated above!) there's probably several hundred species in the Ultra Space universe or dimension.
I totally understand people who have the perspective that they are unlikely to add totally new species to this generation in the middle, and I share that reservation - I don't think that they're going to radically alter every established norm or tradition within the confines of this generation. However, I'll echo the sentiment that Ultra Beasts are - while still Pokemon - something altogether different, something we aren't used to. There are so many possibilities with the idea that there's another dimension, universe, or world with entirely different Pokemon that we've never seen before, and that behave in such a strange manner when accidentally exposed to the world we're used to.
I might not be quick to subscribe to some of the crazier theories - such as the one that Ultra Beasts attacked the "old world" of pre-Mega days, or something - but their presence allows for so much creativity that I wouldn't count anything out, even knowing we're in the middle of a generation.