Looking back at all the possibilities, it's almost hard to decide if any should return because of the possibility of ruining the fantasy I'm sure we've all made up of our off screen characters. I mean, in my head, I see Misty being a totally kickass Water Gym Leader but they could return her with some crisis and she's just a trainer again. There's a chance that a return would be a letdown for all my expectations for these characters. Although, I do obviously want to see them returning for a plot arc or an episode return, just to catch up.
Ritchie. Honestly, he was probably my most favourite rival of Ash's, because as said previously by Kenshin5, he had very similar qualities to Ash and their teams were almost exact. I loved their relationship and it just seemed like it could've continued to a recurring thing. I've always wondered if things would've been interesting if Ash has a reoccurring rival like Ritchie who popped up in a couple episodes but we saw more of him during the leagues. Something constant and interesting while still bringing in new nastier rivals, I just think that a friendly rival wouldn't have hurt. (I'm sure we had a few but yeah) I'd love to think that Ritchie is doing brilliantly and his journey as a Pokemon trainer is going swimmingly. Perhaps he's just won the Sinnoh league? or he's apart of the Battle Frontier? We don't know but sometimes the fantasy and the speculation is enough because I wouldn't want to be letdown and find out that he's not really progressed since the last time we saw him.
Max. As we're all aware, time is legit stuck in the Pokemon world but I would love it if the writers aged him up like they did with Gary and give Max a really great return. I was honestly hoping that the rumours of a new lineup for B&W was going to be a returning character like Max because although it's not fresh or no, but it would've drawn in a lot of old fans and newer fans would've dug it too and could've prompted them to rewatch seasons when Max was younger.
I really do think that having old and beloved characters return as an homage to older fans who have stuck by this show for 3234325436 years. I think it's really imperative that the writers don't forget that these older characters are very significant and important, even if they were just story arc based characters. I think Pokemon Chronicles was essentially there to keep the love for character who have left the journey and essentially keep them alive. Sadly, that's poofed and I really think it's great for continuity to keep the past alive through mentions and guest appearances. Yeah, I'd love it. :D
Well, going by main characters, the likeliness of them reappearing as a guest character, if not a main character, are as follows:
Tracy's a definite. Since he's working for Professor Oak, its a given that he'll at least cameo whenever Oak talks to Ash.
Misty is pretty likely to return at some point. After all, she has revisited Ash at various points, and as demonstrated with the Mirage Kingdom arc, she is fully capable of going to a new region if she so chooses. In addition, she currently isn't following through with her goal, so she doesn't have to diverge from it.
As for Brock, he's also pretty likely to return. Probably slightly less likely than Misty due to being a doctor now, but he's a bit more recent.
May and Dawn are much less likely to return, seeing how they are currently pursuing Contests in other regions (Dawn has Hoenn, and May presumably has Sinnoh to worry about), and since Unova does not have Pokemon Contests, its unlikely they would break from their goal just to see Ash (even May only visited Sinnoh because of a inter-regional-use ribbon being awarded at a contest). In fact, between the two, Dawn's actually more likely to have a reappearance, and that's only assuming that Ash ends up revisiting Hoenn and she hasn't finished her contests yet.
However, even May and Dawn have more than a chance than Max, since, thanks to time being a standstill in the series, he can't age, so he can't re-encounter Ash as a trainer. Then again, he probably has a bit more of a chance than May since he could technically reappear if Ash revisits Hoenn, and that's a very BIG if, anyway. Come to think of it, with May, and all of those factors (even a Hoenn revisit will not have her be likely to reappear, since she completed that region), May's chances of reappearing are the same as a Snowball's chance in Hell.
So in order from most likely to least likely:
1.Tracey
2.Misty
3.Brock.
4.Dawn/Max (had to tie them because of the common factor of the only chance at a reappearance being a revisit to Hoenn)
5.May
