To me I cant see them moving to Civ 5 yet. Technology wise nothings that much better than it was at Civ 5 release date for this kind of game. You really want a Civ 6 to be a leap forward and right now there isn't new graphic, sound tech that would justify the effort to develop. I'd milk this cash cow and when there is tech that really is in front of whats in Civ 5 then develop Civ 6. I think touch screen and the new eye movement tech or kinnect style movement from Microsoft (for X-Box) are in early days. Civ 6 needs to have that sort of tech at a perfected level before it becomes worthwhile using something like that.
With the new Civs debate there are clearly about 60 options that are feasible total in terms of Civs which means they could get two new releases out of this over the next couple of years and then develop Civ 6 for release in 3-1/2 to 4 years. That would really be end of life of Civ 5. So Civ 6 targeted for Christmas 2016 would allow them to start design planning now whilst maximising the earning potential of the current product. That also means in all likelyhood Australia, Majapahit, Moors, Kongo, Hungary, Inuit, Argentina, Sumeria, Hittite, Vietnam, Timurid, Phonecia and a slew of other civs get in making the community satisfied even if those civs miss out now.
2 extra releases = happy community + money for Firaxis + hopefully a Civ 6 using exciting new tech which means Civ 6 sells more rather than just being a slightly better version of civ 5
Expansion wise maybe they do a ancient civ themed pack with more and a modern pack with upgraded future tech modern era options.