To Stormblood.
Going up to Heavensward has been extremely beneficial to FF14 but with the market getting more and more crowded it's gonna get harder and harder to get more new players. Expanding to 2 expansions would be a really big carrot on the stick to get them to bite. Stormblood also introduces a lot of stuff, like Eureka and Bozja, that could greatly benefit from a flux of new players.
Course, like with Heavensward, I do see most players becoming paying players, if they can, before even finishing the expansion or post-game of the expansion. Since if they get through the Stormblood MSQ then odds are they're severely invested into FF14 at that point.
So it's really a win-win situation for everyone just like making up to Heavensward free was a win-win and has helped FF14 continue to grow even as other MMOs struggle to get new players or even retain the ones that they have.
Edit: - I meant with the release of the next expansion, not 2 expansions down.