Exactly. Many people seem to think that one enemy has to be the exclusive one, mostly because that is the normal format of the expansions. The original game didn't really have an exclusive enemy, Burning Crusaded added it by putting a lot of the focus on demon and the Burning Legion (Illidan sort of being an extension of that), the Scourge being the focus in WOTLK, and the Old Gods being the focus in Cataclysm. While the Old Gods had some large importance in MoP, most of the focus was on the faction war and Garrosh.
Blizzard has said since before the announcement of MoP that the next expansion after MoP was going to really have the junk hit the fan. We have already faced the Burning Legion on their own (Burning Crusade), and the Old Gods on their own (Catalcysm being the largest focus). What if the next one be have our enemies both above and below working together to smite us? The Old Gods minions rising from the earth while the Burning Legion falls from the sky. It would be an epic moment.
As for the "Dark Below", I wouldn't worry about all the zones being under water. I have a large feeling what the are going to do is make the "Dark Below" a sort of underworld realm, similar to the Underdark of Forgotten Realms fame. They wanted to do something like that back in WOTLK with Azjol-Nerub, but the zone turned out too much to handle. They attempted it again with Deepholme, to much greater success. It seems about time they take what they learned and have a huge "underground" continent situated around the Maelstrom. It could even be the first real "vertical" continent, where there are different levels going up and down. Maybe one zone is the islands around the Maelstrom, maybe another zone is the oceans, then a cavern that leads into the real Dark Below. There is a lot of stuff that can work well if they grab it.