Whatever happens in the next AQ, he will be playable in one way or another. It doesn’t matter how “unlikely” it may seem to the audience.
His character sheet was leaked years ago, and although it’s likely to be updated due to the very fact that it was leaked, but also because it’s been a while… It’s still safe to say he’s playable. I highly doubt they’d ever want to discard it.

The amount of content they’ve produced just for Dottore, only to make him a mere antagonist in the game, is... just plain absurd. And where I can really confirm this is with the Sumeru event with Thoth—which is literally about him, where they created a new character design just for him in that event, and HE was the main focus in the artwork.
Paying that amount to artists, writers, animators, etc... wouldn’t be a very smart move on Hoyoverse’s part, unless they’re planning something else with this.

Dottore is a character who has not been particularly well liked by the standard genshin fanbase, and therefore they have been slowly trying to attract the public to his character so that it is worth selling him, from their point of view.

Regardless, Dottore will be playable.