I think we can expand are creativeness

what if we could customize the number of people in a offline gp it would be nice and allow more stuff

You can do that actually, but in versus mode

I want it to be on offline grand prix

It is not possible. The goal of the Grand Prix is to be the true mode. VS Mode exist so you can do what you want within the game.

As Grand Prix mode tests your skills and have an important value in the solo part of the game, as it unlock multiple characters, it would not make sense to allow you to modify it as your will.
Like, you could just put 1 single opponent, which would guarantee you enough points to unlock most opponents.

that makes sense