If anything those flashbacks to Brock character when Dawn appeared could be foreshadowing toward his eventual return. There is also fact that Brock still didn't got usual cameo characters usually get when they leave cast(sadly its one time thing never to be seen again), and since Brock is present in BW2 games coming for world tournament increase his chances of appearing even more for advertizing purposes as well.
For that reason i could see Misty having small chance to return too, if expansion of world tournament happens. Like Senior Cup and other advanced tournaments? It would be cool if both returned at same time, but eh its doubtful.
Moving on from that, i could see Brock returning after league in filler arc whether its set in Kanto or Unova making trip to learn about new pokemon native to this region , new ways of healing and gathering knowledge from other experienced doctors which could help him in future studies.
It would be nice to see how he would interact with new characters too, such as Cilan. Cooking showdown to determine who is better cook would be hilarious to see, lol.
I hope we will see Brock battling at least once more. He rarely battled and there is impressive arsenal of pokemon he collected during his travels.
As far as return to main cast goes, while i wouldn't mind i doubt it. If writers wanted to keep him they wouldn't replace character again.
Maybe if they decide to reunite original trio in last season to have things come full circle with group who started anime in first place i could see Brock joining cast for one more time. But that's questionable too.
