1. Neko is already going after Perospero, so the fight between them has already been setup.
2. All the SHs are facing off against members of the Beasts Pirates, it wouldn't make sense for Chopper to be the only SH to not fight a member of the Beasts Pirates.
If Chopper does fight someone it's either going to be the Numbers or Page One if he comes back.
The only way for Chopper to fight Perospero is if Perospero defeats Neko and then we have Chopper fight and defeat Perospero, but the problem with that is Perospero will probably be severely weakened and nerfed due to his fight against Neko. So if they do end up fighting, it won't really be much of a fight between them.