It was revealed during V6 beta by testers that we'd be facing Amber. Although most people assumed we would be anyway, so that wasn't much of a spoiler. Right now though when 7/9 of the remaining leaders are unknown and the 2 that are known are implied to be high up in rank (meaning, most likely not next), I think it would be a huge spoiler and may lessen the challenge if we know in advance.
My gym thoughts are: V8's will be Ground, and the gym after will be Grass. (it was mentioned there were flower fields right before GDC, and I think Aelita said that's where the Grass leader is?) GDC, which might be right after that, will probably have the Psychic leader given what we know about it.
I also think we'll battle both Kreiss and Volta at some point, for friendly battles since we have their badges already.