Speaking of Unova while it tries to carry out vibe of "fresh start"following games interpretation of said region being far and very isolated from others Ash traveled through that doesnt man how Dawn and Brock doesnt have any chance to appear in it.Anime is still treated as continuity despite lack of more references toward past as it can be noticed with older characters beside Ash like TR,Giovanni.prof.Oak,Dehlia etc still existing in anime timeline and recent mention of Tracey clearly shows how ties to past havent been cut of yet.
Argument of Unova being far away doesnt play any saying in Dawns or Brock return either as long writers would create something which would be beneficial for their stories and plot of current series.Visiting Ash would be just additional benefit with primary purpose being to advance their dreams and story.
In reality it doesnt matter how much some region is distant if writers create something which would justify their journey over there.
Misty return in Hoenn and Mays in Sinnoh proved that.
Yes Brock needs to study to become doctor but its not known yet if he plans to become traveling doctor or stay at one place and i can see him going to Unova with purpose of expanding his knowledge about pokemon or learning from other doctors which could help him in future career.
As for Dawn i could see hr coming back to either play some role in Team Plasma arc helping to fight against it like Gary in DP did along with perhaps partaking in on of Don Gorge tournaments.Or she could go there to try promote concept of contests among people in said region since they dont exist there.
So while not very likely considering recent play of things in this show Dawn and Brock might have shot at appearing in Unova still not receiving regional cameo appearance like others in past did.
Sam goes for others like Misty,May etc.,They have chance to return as well having potential to offer something on table.Especially when taking in account how some of them havent got closure to story and goal with lot of material being left unused.
Gary was not a main character. And we knew he was going to appear in DP because the last ep of AG told us he was going to Sinnoh.
It would have been pretty weird not to see Gary in DP at all when he said he was going there you know.
Your forgetting how Gary stopped mattering to show long time ago but that didnt stopped writers from making him relevant again creating purpose behind his appearance at end of BF being transferred to Sinnoh.
If minor character like him was revived again,there is no reason why someone more important and with more potential to offer to storyline like ex main characters couldnt return as well.