Reason: I'm dissapointed that just about all of the poison type specialists to this date are ninjas, and from kanto for that matter (excluding Roxie, but I havn't fought her yet)
If I were a poison type specialist I would take on a different theme, possibly a scientist or medical doctor or maybe the head of a (nuclear?) Powerplant! I've got it!
The champion of a region being the rich son (kinda like steven) of a powerplant owner. Growing up, his fathers powerplant has brought in a lot of poison types to where he lives. He's made them as his friends and now he's the most powerful trainer in the world! If that were me my team would be:
It's well ballanced because it helps cover against poison's natural weaknesses, psychic and ground.