To be fair since there exist something as traveling doctors Brock could decide instead of opening daycare center to go on journey to learn more about pokemon and utilizing his knowledge outside by haling pokemon and learning from other famous doctors.Why would a doctor travel in a group of 3 or 4 24X7? Brock's right place now is pokemon centre since he is a pokemon doctor. I was quite sad to see Brock switching his goal out of the blue after 14 long years.
But the writers have decided to make him a pokemon doctor anyway. I just don't see him returning for another adventure with Ash. It just doesn't make sense to me.
Since we dont know yet what kind of path Brock will choose,in case of becoming traveling doctor having him travel in same region Ash headed to joining him on journey to learn more would make sense.
However since writers already brought him back once when he mad break during OI just to replace him once again i highly doubt they will give his character yet another chance in this show.
There are replaced characters who deserve to return more than he does getting a lot less,but if hypothetically speaking Brock again becomes main character i will be fine with it as long writers provide plausible reason doing something worthwhile out of it.