User blog comment:Trainer Micah/Ash's Past & Future Pokemon Capture & Released/@comment-32664920-20191119173745/@comment-38149523-20191120061810

Trainer Micah, I don't think that Ash would catch a Piplup considering on how Dawn already has one and it is already iconic in the anime. Then again, one may never know.

As for my guess, maybe Lucario considering on how it was shown on the poster and there is a sticker set featuring it for the anime.