I think the Old Gods will always be more of a sub-plot, side quest matter, rather then the main focus.
The same way that N'zoth was involved with Deathwing, Yogg-Saron was behind Ulduar and Y'shaarj in Mists of Pandaria, none were major focuses of their respective expansions, but they still had enough involvement to be interesting and engaging. I think an expansion totally based on them would be rather dull.
Old Gods are exceptionally powerful, and killing off the remaining few is going to prove very challenging. I don't believe we know where most are, and I don't think it's possible to fully destroy them with our pathetic and weak mortal weaponry. It'd be a bit of a slap in the face for us to be able to overcome these immensely powerful beings in a mere two years, or when the next expansion launches and we're suddenly battling against far weaker foes.