Originally Ash was going to have a talking Clefairy, which was honestly a very odd choice... so I am glad they never moved forward with that idea.
In most of the sagas, Ash always has one pokemon that he bonds extremly well with. If not Pikachu, Charmander would have been Kanto. Johto once again focused more on Pikachu that others, but I feel like he bonded more with his Donphan. So that could have possibly be he pokemon in Johto if that is where he had started. Hoenn it was Treecko, so that would have been his choice. Sinnoh was Chimchar. Unova was probably his Krookodile, Kalos was obviously Geninja. Unova was Lycanroc. And finally Journeys was Lucario.
I think over all, Riolu should have been his starting pokemon. Maybe it didn't evolve for a while, but that would show his growth with Lucario. Lucario went through an entire sagas worth of development in a season which I didn't like. Maybe Riolu would have finally evolved in the Unova League like Cameron and then that bond could have carried over into Kalos where he got Mega Lucario or even a Battle Bond Lucario. And then got a new form in Journeys. Ash has always had a strong aura as shown in the Lucario movie, Sinnoh series, Kalos with Greninja and Journeys.