I think premades will come down to guard more so than anything else, imo it's the strongest PvP ability right now. Second to that would probably be stealth detection. So I see a lot of successful premades being composed of commandos/mercs (stealth scan is best stealth detection atm) along with probably shadow/sins for guard.
But really the best premades are those that are more flexible. I don't know if the 8man teams are going to be forced to be static, but if you can vary your roster freely than those teams that are willing and able to play different classes/specs are going to be those that are more successful.
People will play what they think is the best to begin with, but slowly some strategy or team makeup will become popular, like multiple Marauders for force leap. The team who's willing and able to field multiple gunslinger/snipers to counteract this is going to be the most successful, at least until another strategy becomes favored. The teams who have multiple 50s to choose from, and the intelligence to field those 50s against specific tactics, are going to be the most successful long term, regardless of initial makeup.