but their rivalry was still there, and I remember saying a new form as Greninja did rather than a Mega Evolution.Yeah, see, something as big as Lucario getting a brand new form warrants complete ace status. Those two you mentioned ended up being side dishes for the main attraction that was Pikachu VS. Koko. Dusky was lucky that it got a hell lot of Grand Trial focus (despite being just two battles in battle-starved Alola), and Torracat was lucky it was even relevant through Kukui.
However, in the event that Lucario only gets Mega Evolution, it won't be quite enough to give it an ace card with G-Max Pikachu being a thing, and the games won't do a third version because the DLC was the third version part. Even if there are new forms to use in Pokemon Legends Arceus, it's hard to see Lucario being the ace when we're already promised a fight between Pikachu and Charizard (this being an alleged worldwide tournament simply can't have Ash win the big matchup with some other Pokemon other than Pikachu).
...Also, Pikachu has hogged a tremendous amount of spotlight that it's a bit random to make an ace this far.