This thread title reminds me of Rugrats All Grown Up! ;)
Ash: Ash will really mature a lot, but still have the same determination, and love for Pokemon. Of course, he will still have Pikachu too! He will be Pokemon Master, and will be famous, almost definilty. He may calm down and focus more on his future ocupation, but, he will be the same Ash we grew up with.
Misty: Misty will surely tag along with Ash, until the end of time. But, since she got her bike back, she will probably be asked by Ash to join him. She will probably become less of a tomboy, and want to be pretty in front of Ash. She most likely won't admit her love for him, because her temper will always cover it up for her! I think that she will also still have a passion for water Pokemon, and get her Togetic back. Overall, she may change a lot, but still have parts of her childhood would shine through.
May: May will probably travel on her own, or with Drew. She will care mostly about the appearance of her Pokemon, and sometimes leave out some of her Pokemon. She will keep in touch with her family and friends, because she is the kind of person who couldn't live without friends. May will keep winning contest, and will always be a little stubborn if she doesn't, but her wins and loses will help her mature.
Brock: Brock will mature a lot. He will still be attreacted to women, but most likely will settle down with another breeder (whose a girl XD) I can imagine Brock owning a breeding salon, and will be nationaly known.
Max: Max would most likely be a Pokemon Professor, studying in the fields like Professor Birch. He would be over his bratty know-it-all stage by then. Max is very smart for an 8 year old, so he will most likely be a well known professor.
Tracey: I get back to him...
Jesse: Jesse will still have James with her, and she will be the leader of her friends. Still having the anger and fiery personality. She will still think she is beutiful, and will do anything to get a promotion. I can imagine Jesse being leader of TR, but, we'll have to see on that!
James: James will still be the same, maybe mature a tiny bit, but, he's always going to be the funny James we know and love.