It's certain that Hikari/Dawn isn't going to make a cameo in Isshu/Unova. Neither is Brock, May, etc. Older characters isn't going to even mentioned at all in Unova.
You and the 99,9% of the fans knew that Dawn and anybody (except Looker) is going to make a cameo in Unova. All of this is very true, true.
All of this are true for BEST WISHES SERIES. Not BW2 series. The key reason because the writers identify season 2 if it's going to be a new saga is because BW1 had obvious limitations. And i'm going to resume some of these:
The only characters that can appear from older sagas are Ash, Ash's Pikachu, Jessie, James, Meowth, and her direct relatives (Delia, Oak, Giovanni). Nothing more.
The only Pokemon who can appear are Pikachu and Meowth. The rest are only Unova Pokemon.
The goals that Ash had obtained in previous sagas can't be mentioned. Only it is mentioned very barely in BW001 for the viewer setting the character.
The series is treated if it's going to be Ash's first saga. (for example, if now Ash mentions that he participated in four leagues to Trip/Bianca/Stephan they will be shocked).
In Season 2 all of this is broken. I don't think that all past characters are appearing in the saga, but I think that more older character cameos will happen. Past characters that may appears in season 2 are Brock, Gary, May, Misty, Paul (yes, for all we known he makes sense to enter the Unova League).
And, last. I think that if for some reason (highly unlikely Dawn won't leave at the end of Junior Cup. I don't think that the rest of her Pokemon are being showcased. If anyone hope of episodes for the rest of the team after the Junir Cup, I think that it won't happen.
If for some weird reason Dawn is recurring or main character, she'll probably reset her team except Piplup(obvious) and probably Buneary (BW2 promotion). The rest are going to be Unova 2 Pokemon.