on the matter of sargeras it seems they could fit him into just about any expansion quite easily.
"As Aegwynn surveyed the scene, she found the rift from which the Legion's agents had come through in the Storm Peaks. Using this portal, Sargeras had entered Azeroth to confront Aegwynn. The Guardian of Tirisfal did not hesitate in attacking, but Sargeras held back, letting the mage destroy his physical body. However, as it died, Sargeras’ spirit reached inside Aegwynn’s body and hid, lying dormant for many years. Unaware of the tainted presence inside her body, Aegwynn buried Sargeras’s physical body in a tomb and cast it deep into the ocean."

"Interesting enough, according to additional lore, it is said that Aegwynn may not have fought Sargeras' original body, but rather only fought an avatar of Sargeras. It is said that this avatar was infused with a portion of Sargeras' soul which he controlled. Medivh was born with this fragment of his soul, and it later escaped back to the Twisting Nether.Brann Bronzebeard is pretty confident that the real Sargeras never actually set foot on Azeroth, implying the real Sargeras is still around in the Twisting Nether.Lending this theory further credibility is in War of the Ancients, the power of the Well of Eternity was needed to create a portal strong enough for Sargeras to attempt to enter Azeroth, the creation of which took a long time to be completed. The portal used to bring the Sargeras that Aegwynn fought was powered by a handful of demons and was created in only a few minutes.

Brann also believes, along with others, that the portion of the spirit that escaped still endures, hungering for revenge."

Because of the ties to the Lich King and both expansions as a whole I could quite easily see Sargeras play a major part in a future expansion.