Brock's Return & Dawn Not-so-much
Let me say that I don't see a reason for Dawn coming back except for fans. I myself love Dawn, but I think the writers wanted the Sinnoh Saga to end. May got a bit of a closure and the only reason she came back in my opinion, was that people were curious. However, Misty didn't have that exact closure. So, I can see that Misty would come back, but that's not what we're talking about, now are we? However, I can see the possibilities of Brock coming back.
My Own Personal Intake - (As in, looking through my own feelings like all of you are! :3)
I can see that Brock comes in and he is an intern or a doctor still in training. With all the Pokemon Centers and the Nurse Joy's that seem to be new to what they do, it would not be much of a surprise that he helps out. That and I'd SO love to see Brock and Dento go at it a mock Iron Chef showdown. Those by all means are my personal opinions. I also want Brock to have a NEW voice actor. His second VA killed Brock. The second to last episode of the Sinnoh Saga when Nurse Joy was all, "Brock, you have to calm down!" - Yeah, there was NO expression. Now I'm just rambling and ranting. Lawl.
A Critical Perspective - (No feelings, just a lotta thinking x_x )
The last we saw of Brock was when he left in the last Sinnoh Saga episode from the post that pointed to Pallet Town and Viridian City (or Forest). Take into consideration that whenever Brock did leave, he sort of came back. That could clearly be a slight end, but he would have an appearance where he meets Ash's new friends. He'll come back as a special when Ash is back in Kanto or he is visiting Unova. But the only reason he could ever come back otherwise is to have a lot of viewers. If Brock were to come back at all, he'd play no role except for that one special episode where he just so happens to visit out of the blue to please fans.
My personal intake on who should make an appearance -
It's time he met Ash.