Who is to say that they wouldn't give Ash Grookey or weak Pokemon? Riolu literally hatched and is training to be powerful. Same can be done with that weak Pokemon and why putting Grookey under same basket as weak Pokemon, what if Grookey was strong? Also what if he undergoes fast evolution to become strong Rilaboom. Precisely he has Riolu and Farfetch'd that doesn't mean he's regressing all the time.
Riolu has already taken the baby pokemon that is weak and grows to be strong slot on Ash's team. Grookey would be another pokemon which would again require Ash to fight train up from scratch and mind you, the pwc is not like the old regional league. What is he going to do once he starts matching against E4 l level trainers higher up in the ranks?
I agree that it's very unlikely they would bring back Ash's entire OG team for the battle against Leon. That's more something they'd do in AG (exactly what they did with Brandon) and not now, after so much has passed in between.
But I don't agree that Ash would use an entirely newly acquired team, unless he already uses a lot of reserves on the last couple matches leading up to Leon, then I can see it happening (like how the DP team got Paul's battle in Sinnoh, the most important one, while the reserves got their share on other battles).
Also, why would Ash bringing Charizard back for this battle make no sense? The fact that Leon's Charizard is the strongest Charizard in the world doesn't mean anything, cause Leon's Charizard is also the strongest Pokemon in the world, except for the strongest legendaries. So Leon's Charizard is stronger than literally any pokemon Ash could bring to that battle, thus the statement that bringing his own "inferior" Charizard wouldn't make sense... well it doesn't make sense.
Besides, from a plot standpoint, Ash's Charizard was left training in a Valley with all the strongest Charizards in the world, and by this point he's surely the strongest one there. So it would make sense for it to want to test it's strenght against Leon's. I'm sure that if the writters still cared about continuity with the series as a whole, they'd do exactly that actually. Even when it's Pikachu who defeats Leon's Charizard in the end. But it doesn't even have to happen that way really, Charizard could just battle different pokemon of Leon and not his Charizard.
And about the Grookey argument. I see your point, I also thought Ash would get only strong battle-ready pokemon, but then Riolu and Farfetch'd happened. So I think it's quite likely his last pokemon will be one that needs training before becoming stronger too, like Grookey.
It could be Sirfetch'd too.
You seem to have misunderstood what I said. Let me try to better explain.
Every 6 on 6 Pokemon battle usually revolves around 1 main battle. The 2 main Pokemon in this particular battle would be Charizard and Pikachu. If Ash was bringing in his Charizard to challenge Leon's, then it would make absolute sense for him to do the same. However, that scenario is extremely unlikely. Hence, if Ash uses an inferior Charizard which battles some of Leon's weaker pokemon and loses, following which, we have the final battle between Charizard and Pikachu, it take away the spotlight from Leon's Charizard.
In other words, Imagine if Gary had a Blastoise and a Charizard instead of his arcanine, during the Johto League. Assume Gary's Charizard lost to Snorlax during the battle instead of arcanine. This would've taken away the spotlight from Ash's Charizard(Which was his ace during the battle).
Or for example, assume Alain and Sawyer had a Greninja or if Paul had an infernape(which was weaker than Ash's infernape and lost to one of Ash's previous pokemon).
From an anime perspective. A duplicate vs duplicate pokemon battle only works when both the pokemon, hold the same place in the respective teams or have a form of rivalry.
With regard to some of your other comments
1) How exactly do you know that the Charizific Valley has the strongest Charizards in the World? Leon's And Alain's Charizard are far stronger than Ash's and they both didn't come from the Charizific valley.
2) Your second comment about "writers caring about continuity and wanting to put Ash's Charizard against Leon's", simply comes out of Nostalgia. Ash's true ace and first Pokemon is his Pikachu. While Ash has had several aces over the years which have shown to be stronger than Pikachu like, Charizard, Sceptile, Ash Greninja etc, I believe this final battle which gives Ash a chance to be the strongest trainer in the world, should come down to Pikachu. That makes more sense from a story perspective than your take about having Charizard fight Leon's.
