Answer is all of them.
Let see:
-Misty:its a shame how much this great character is being ignored like she never existed definitely deserving some screen time getting update of her and potentially receiving more development advancing her story which was left unfinished in many areas.
Scenarios behind her shorter return could be:
-to come help Iris about potential problems of becoming gym leader with Iris learning something from her(similar to how May helped Dawn)with possible connection between them hanging up in the air having some things in common
-coming for some water tournament to advance her goal and skills as trainer would also make sense
-coming to catch some new water types which arent available in Kanto(due to Unova region being very far from other regions thats entirely possible)also sounds plausible,
-deciding to meet some powerful water trainer wanting to learn something from it
There are more ideas,but i narrowed choice down to this four for now.
In reality Misty should return to main cast due to unfinished story having potential and many unexplored thing about her(like past,goal etc) to see her develop more as character bringing some humor and good chemistry within group but some decent cameo of 4 to 5 episodes would be satisfying for some time too.
-Dawn:just like any other character before her who got cameo during region its fair that she gets one as well to inform viewers how is her next journey going and what has been happening with character in meantime.
Possible scenarios behind return:
-to come to try promote idea of pokemon contests in Unova region among people to get them familiarized with different style of battling in case contests dont exist in that region.
-to come for some tag battle tournament which fits her character considering how she already took partake in different kind of events aside from contests in DP.
-Brock:for same reason as Dawn he should appear to.
Possible scenarios behind return:
-travel to Unova for doctor purposes to examine new pokemon and do some research about them
-to hear about some famous pokemon doctor who lives in Unova deciding to travel over there to meet him and gain some knowledge about it.
May-because im interested in seeing some update about her and how far she came in Johto Grand Festival.Judging by her performance and pokemon team in Wallace cup its not unrealistic to expect her winning whole thing finally.
Scenario behind return could be same as Dawns in case later doesnt appear.
Some people mention pokemon musicals as possibility to have coordinators involved too,but im not sure how well that could work.
Nobody in Unova. Nobody should make the long trip over to Unova as there is no reason for any of them to go there at the moment.
If there doesnt exist reason writers can always create some to have them return in Unova.Distance has never been a problem.As we get to see May didnt had problems with delaying her journey through Johto to com for Wallace Cup for example.
If anything in my opinion Misty and Iris would make sense to meet each other(more than it was case with Dawn anyway) having some connections and potential to benefit from their meeting and Unova being isolated from other regions make its pokemon hard to obtain outside of it(including water type too).
To
TurtwigFan1 :Several TV stations like Boomerang,pokemon Smash,Disney channel,Cartoon Network etc air or were airing recently reruns of past series like Kanto,Johto,Hoenn etc.with many today kids watching them.
On top of that today kids could have heard about past sagas from older relatives,through Dvds,internet(we are in 21st century with this form of media being accessible for kids at their youngest age already).
Proof:Certain number of members is between 9 and 12(which is in range of target audience surprisingly being well familiarized with older series.
Im only talking from my personal experience but i met some of today kids and they definitely know who was Misty or May watching older series liking them just as much as they enjoy in new series,some even more.
Also i find it surprising how many of Misty or May fans on pokemon online boards are under 15 which says alot obviously not being among pokemon veterans.
