I think that during the filler, Dawn will leave to travel around Hoenn, with Nando and Zoey, and seeing how strong Piplup is and that it could use a lot of training, gives it to Ash to train.
I could definitely see Dawn going to Hoenn after the series, but I don't know about giving Piplup to Ash. I mean, it's her starter Pokemon. The only crazy thing that I could see possibly happening, and please don't flame me, but I think after Ash kicks Paul's *** in the League, Paul might realize that he's training his Pokemon the wrong way and change his ways. Then maybe Chimchar/Monferno/Infernape will rejoin Paul. Very unlikely though.
Here's what I think:
Ash:
;025; obvious
;388; I really want Turtwig to evolve, but it probably won't go all the way
;392; I think Chimchar will go all the way to kick Paul's ***
;398; Staravia will most definitely evolve, all of Ash's bird Pokemon have
;418; Probably won't evolve, but if it does, it won't be until the end of the series or the filler saga
;472; Probably going to stay in the team for a while, I'm glad Ash got it, it's so badass
;447; gets egg from Iron Island, will hatch when he gets to the League, may evolve in the filler
Dawn:
;393; I want it to evolve, but it probably won't for a while
;417; probably not going anywhere
;428; I can definitely see Buneary evolving
;424; probably will stay for a while, but I think Dawn will leave it with Oak before she leaves
;473; why else would she get Swinub?
;470; I think it would fit Dawn, but probably unlikely because they're past the Moss Rock
Brock:
;185; glad he got it, it really fits him, and it fits into the Rock type leader thing
;453; also cool, it fits Brock well, but I don't think it'll evolve any time soon
;440; fits the whole breeder thing, seems to suit Brock, don't think it'll evolve for a while
;458; now that it's been brought up, I think it would suit Brock and it fits the breeder thing
;465; I think it would really fit Brock
I must say that I'm very happy with Brock's and Ash's teams so far.