Till I know how this "including all the regions" thing works and how long he's likely going to be in each region (and how long this series is going to be it could be 3-5 years for all we know) I'm just throwing out ideas for what Ash could do there. At the very least I'd like him to reunite with all his old Pokemon, maybe get a few of them back (Pidgeot, Primeape, Greninja), officially catch Larvitar while visiting Johto, rotate the ones he had in those regions onto the team while he's there, evolve a few, etc.
I'm not suggesting something random like a Deerling. I think any old region captures should be the Top 5 "that got away", you know pokemon Ash bonded with, but didn't catch? The ones that people bring up again and again? Those are the ones I'd like him to get while he's visiting old regions. I'm talking about Larvitar and Riolu.
He shouldn't get anything from Kanto or Unova though. He already has plenty from those regions (and therefore need more attention/focus)
Otherwise yes I agree with you. I'd love to see Ash use Kingler again.
I could see a few being unexpected (or not get anything from) though since IIRC aside from Spoink (which bonded with the group as a whole) Ash didn't really interact with any PotD in Hoenn that was all May and Max.
Yeah.. those pokemon could really work. Like Haunter from Kanto, that Larvitar from Johto, Spoink from Hoenn, Hippopotas from Sinnoh, Venipede from Unova, and maybe Skerlp from Kalos, from Alola I guess the best would be Passimian? I would love to see Ash using those along with the reserves he has.
Also, Ash should re-capture some pokemon he has released. Primeape, butterfree, pidgeot, lapras, greninja, goodra and also ambipom. They deserve to be with their trainer (except maybe butterfree?)
It could be wonderful, if at the end of Galar, we get some major 6v6 battles, at least 7-8 of them. I mean, trainers travel whole year (assuming it's a year and not a week *cough* Ash* cough), just to get knocked out in first battle which is 1 on 1. It is very unfair. All the league matches should be 6 vs 6. Every new pokemon of the region should be showcased before the league starts, and pokemon from all the regions should be used in the league. Some unused pokemon too. Like I would like to see Hippowdon in the battle, and Starmie has never won any non-TR battle on screen, and other pokemon too