The big 4 (Pikachu, Charizard, Squirtle, and Bulbasaur) are probably his most experienced and overall most used of all his pokemon so it shouldn't be surpising if they outdo his all his other pokemon. It sounds like Ash will get inspired in the rival episode to go back to basics and bring the old team back since they where pretty much the team that started it all and pretty much carried him till he got to Johto.
As CC said the fresh start is over and done with. The fresh start was just a cop out for the writers not for him to drop them off along the way and basically let them go like he did with Butterfree, Primeape, Pidegot, Lapras (even though everyone knew it was going to be let go at the end), Charizard, and Squirtle which was very unpopular way of freeing up space for new pokemon. So dumping them all off at Oak's and letting them mostly rot was a better way to do it
I have a feeling assuming that May and Ash return for D/P that they'll both head back home, hear about Shinou in different ways, and bump into each other along the way. They'll probably follow Ash and he'll bump into Hikari, and they'll watch a contest or something that May is in. Not exactly original but when has the series ever prided itself for it's originality
As CC said the fresh start is over and done with. The fresh start was just a cop out for the writers not for him to drop them off along the way and basically let them go like he did with Butterfree, Primeape, Pidegot, Lapras (even though everyone knew it was going to be let go at the end), Charizard, and Squirtle which was very unpopular way of freeing up space for new pokemon. So dumping them all off at Oak's and letting them mostly rot was a better way to do it
I have a feeling assuming that May and Ash return for D/P that they'll both head back home, hear about Shinou in different ways, and bump into each other along the way. They'll probably follow Ash and he'll bump into Hikari, and they'll watch a contest or something that May is in. Not exactly original but when has the series ever prided itself for it's originality