I hope we keep the 6, and we get an omni-warrior, or a melee class of some sort. I don't really care what people say, Omni-warrior sounds cool and would be cool to see how the technology has evolved. It could even be a hybrid melee class that also gets the best exploration abilities.
Instead of a class focused on exploration though I hope each class gets a unique exploration class power, like they get for an offensive power. Engineers can detect places of loot or places of hidden Remnant technology faster. Infiltrators get a special cloak that they can't use in combat, but has a far higher duration and conceals them from detection, allowing them to explore Khet outposts or caves with cool alien lifeforms without being detected. Adepts can destroy, or lift, rock formations to reach previously unreachable locations, or at least make them easier to reach. Vanguards get to combine their charge with the jetpack for crazy height and distance. Sentinals get a hybrid mix between the adept and engineer exploration ability. I have no idea for Soldier though, maybe they can carry more loot (if they go with a carryweight system like in Inquisition even though I absolutely despise those). Omni-warriors can perhaps hack things easier, like a rogue in DA can lockpick stuff.
I have no good ideas, but I would like a unique exploration ability for each.