Which Other Gym Leaders Deserve Promotions to the Elite Four?

Pokémon Gym Leaders are some of the most prestigious trainers around; but there's still the Elite Four over them as the most impressive Pokémon Masters! However every now and again we get to see a Gym Leader rise up to the ranks of the Elite, so which other Leaders would be the most likely to see a similar promotion in the future? (Especially from the regions where we've yet to see them...)