I want this generation's Gym Leaders to train a diverse range of types. The one thing I'll really crack it at, is if the first gym is a Rock Type Gym. We've already got tired of this type, and I think it's time for something fresh.
A Dark Type Gym Leader is definitely needed, no doubt, zero doubt, period, end of story. What I'm even more interested in however, is if the developers have finally understood that us gamers want more of a challenge. And not in the form of a silly Challenge Mode that's unlocked at the end of completing the main plot, (which in itself only raises trainers' pokemons' levels by 5 or so). We want a challenge straight off the bat. Meaning, we want Gym Leaders with their each, own, individual strategies.
I want so see each Gym Leader have a battling technique that complements the type of Pokemon they're using. I want to see each Gym Leader concoct a strategy to cover their type's weaknesses. I want to see each Gym Leader become smarter and stronger than the next. I want each Gym Leader to make us think, make us win by forcing us to concoct our own battle style, our own counter-strategies, not just through type adv/disadv.
Example: A Steel/Rock Type Gym Leader utilizes moves such as Rock Polish and other speed-boosting moves to help support their otherwise extremely slow Pokemon.
Also with new Gym Leaders, I want harder gym puzzles, and much better badges. Gen V's were horrible imo.