Pick it, it can't be ghost/dark/psychic/water/fire/grass type though (First three reserved for later gyms).
BUT you have to think on where you're going to encounter him (No gym building in the ROM if you noticed and there won't any), I like to fight a ground type gym leader more on a mountain top then in a house.
For that you need his name, a story how you will encounter him before even starting to callenge him, the area where you'll fight him (If he's electrice gym leader he can even fight you in a power plant). You need his signature pokemon, his team, a little story before you met him.
And which is the most important thing in every encounter you will have him the dialogue (You can even make the dialogue "before he knows you want to fight him, after that, if you talked to his mother, if you didnt talk to his mother) and everything you want too :p
Also he has to interact with Dj Lefty, MC middy and Dog righty before you enter the city :)