It's become painfully clear that Ash isn't going to capture every Pokemon [which was my original thought when I heard the phrase, Pokemon Master] Since Ash is growing up...somewhat...I think he should hook up with one of the girls he's been travelling with. I think May is on the same mental level as he is but, Misty would be the more logical choice since most harem anime series make sure the female who ends up with the token penis is the female who kicks his ass the most often. It would be nice to end the Pokemon series the way they ended Harry Potter
[pseudo-spoiler incoming]
Having an adult Ash watching as his first born son embarks on his own Pokemon journey, taking Pikachu and Ash's old Pokedex with him.