I expect from the DLC the following:
-a hopeful great story with the 3rd Legendary. It will be interesting bc we havent in SW/SH the same Situation! I think one whole DLC will play out in AZ and find more about it
-Carbink and Diancie are Transfer only.. i could see them do some sidequests. Think of Keldeo in Crowns
-Volcanion is also transfer only after it was also in Shield transfer only. Maybe also something bc second in a row as a mythical
-Groudon, Kyogre and Raquaza is also transfer only.. this could be interessting. When they decide to do something with them.. what could it be?
- I hope they reuse/reimagine more Megas into Paradox Pokemon. The Feature is great! the Forms are new Pokemon. So we will keep them around and People really really like them like Megas before. Its a very good substitute
- I think the second Part will play in the hidden landmaß. Call it southern Kalos, northern Palde. It will be something like that
- I dont think the third Legendary have a link to Zygarde. Nor Kalos Leggys Yveltal Xerneas will play a role. They would probably already be transfer only.
-Megas coming back? Maybe! its a bit fishy the weather trio(Raquaza etc.) is tranfer only. I think it doesnt look to broken comepetive wise with Teras. I could see it bc Fans still love it and i could see to work it with only Tera beside that accessible. having it only available in the DLC makes also sense to give Tera the spotlight it deserves. (i personally would rather prefer to bring more Paradox with Mega influence bc we will keep that forever)
- I dont see special Forms for Tera Pokemon. i think we would see them already in the basegame or atleast some hints about them.
my 2 cents