Well for Ash that much is known how he is main star of this show with whole plot revolving around him so his long stay isn't surprising.Pikachu is mascot of pokemon franchize and trio of Jessie,James and Meowth served role as antagonists toward main protagonist acting as obstacles on Ash's journey.
If anything Team Rocket current role in BW being more of a threat as villains being accompanied with Giovanni's return indicates how they might likely leave at end of Unova with whole rocket organization getting disbanded.
Surely its still too early to tell because those three have been part of show for 13 years already and could stay past BW too but than again Brock was long lived character too and he still got axed.I guess time will tell.
Suggestion of James becoming pokemon master sounds like forced attempt to keep him relevant to this show if you ask me.For start he has several issues with lack of confidence with his type of character not being suitable for it .Sure there were few references but if anything this series showed us was fact of his character not being sure what to do with his own life trying to find himself with his current role in Unova making it seem like he finally along with Jessie become what he wanted.Turning him in some sort of champion would be completely OOC making it look like bad writing.
I wouldn't mind though if he was kept after TR gets disbanded to find "new calling" taking up some role which would fit more his character like competitions along the line of poke-ringer.Similar to how Jessie did contests going along with characterization,but to turn James in regular trainer doesn't make much sense with his character not being suitable for it.
I'm still reeling from the fact Misty left
What bothered me when Misty left was because i feel she didn't got enough focus during her run having untapped potential to do more.That and her send off was little unoriginal having cliffhanger touch to it(with story being left hanging in the air as hoso showed with character taking care of something because of feeling its his responsibility,not because he wanted).Though from emotional aspect it was one of best ones.
I can see writers maybe deciding to bring her for last season along with Brock with attempt of reuniting original trio to finish pokemon series with group which started it.I suppose this would please many older fans with things coming full circle.
Though i wouldn't hold my breath over it.