Let's assume that they chosen Pikachu as the mascot for the franchisee, because it's Ash's main Pokémon.
So I'd pick a Pokémon from a theory: Clefairy.
I believe everyone here should know the theory that originally, Ash would receive a Clefairy from Prof. Oak, not a Pikachu. But the producers denied the idea claiming that Clefairy was a girlish Pokémon and the boys wouldn't like it. But I'm don't think so, because I'm a boy and I like Clefairy it's cute and it can learn cool moves (well, maybe when I was like seven years old, I really would think that Clefairy is girlish).
So, keep talking about theories, there is another popular one that talks about Gengar be the shadow of Clefable. Leaving aside the creepy part of the history, I think would be nice, if Ash has a Clefairy/Clefable, the Team Rocket has a Gengar and be literally the shadow behind the protagonist (and come on, Gengar is really fun).