I would love to be a fire type gym leader. In nearly every Pokemon game I have played, I have always gone for the fire starter since they always seem to have an awesome second-type that goes along with it. Charizard has flying for his secondary, Blaziken has fighting, so on and so forth. With those combinations, there can be some great move-types that could devastate anyone that would dare challenge me (unless if they were smart and brought along a water type).
Now, I know I couldn't go for just starters, but I know I would have some, what I like to call, Contradiction Pokemon, being Pokemon with opposing types, such as Numel or Magcargo, since they would be harder to take out with just ground and rock moves that I know a lot of trainers would have to take me out.