I will be honest with one thing, I do not want Ash to get Mudbray, because I know that it has High Horsepower, but let's face it, we all know how shortchanged the ground types get in this show, being unable to access their best moves(Heck i'm still not sure if we'll see Tectonic Rage more than once), and imo Rock and Ground are too similar to each other in terms of type, if Ash had not caught Rockruff, I would've gladly accepted Mudbray.
Another problem is, i'm not so sure Ash would be given a ride pokemon, basically something he can ride on, on a daily basis, I know Kiawe has Charizard, but he has a good reason, given he lives on Akala where as the rest of the class is on MeleMele and needs transport, also we all know he uses Turtonator and Marowak for battling purposes so much that his Charizard has never been used in official combat, and then there's the fact that while Ash has 30 Tauros (because I know someone's bound to bring this up) , he's never used them as permanent team members, or rode on them, they've always been reserves.
If Ash must capture a Alola pokemon for his last slot, then let it be Rimbombee(granted Ash has ran across both forms of this multiple times and hasn't shown interest in them, so i'm not sure) or Kommo-o, otherwise bring Greninja back(Let me remind anyone against this idea, Ash-Greninja wasn't obtainable in gen six, and personally i'm tired of Lycanroc getting all the attention), or have Ash get a regular Kanto pokemon to premote the spin-off's