This link from Serebii has a list of all the Pokemon Ash had and still has so far:
https://www.serebii.net/anime/characters/ash.shtml
You know, all the Pokemon at Oak's Lab are still with Ash indirectly, since he just stores them there and can withdraw them whenever he wants.
But from the Pokemon that he has actually left behind earlier, I would like to see a one time appearance of Butterfree and Haunter.
And I want Charizard, Squirtle, Pidgeot, and Primeape to return to him permanently. They had enough training, and Pidgeot had enough protecting the forest from Fearow.
Specially Pidgeot. They could make it so that Ash goes back to Viridian Forest and this time captures Fearow, giving Pidgeot no other reason to stay behind and re-join Ash since Ash promised.
That's because the Fearow is a considerably important part of his journey as a trainer, and how it all started.
It has been shown that Ash already wanted to capture Fearow last time in Viridian, but failed. I think he'll succeed the next time he tries.
This is to imply a character growth in Ash too, and imply that Ash is now far better than those days.
Ash already met Lapras another time in another episode, and he also had a nice goodbye with Lapras the first time, so Lapras doesn't need to appear again, though a one time appearance wouldn't be bad.