New expansions come out towards the end of every 3rd month - so the next one'll be end of June. The initial trailer usually drops at the start of the month, with sequential card reveals in the month leading up. That being said, the next "expansion" is actually a continuation of DBNE, with each class receiving a new Leg and Gold, which comes out late May.
Assuming you're new, just go ham on DBNE and Chrono. As busted as Tempest is, it'll be gone in 2 1/2 months, and for a player starting out it's not worth the long-time investment to spend precious rupies on the pack. Craft whatever cards you need from the pack. The most recent two are, well, the newest, and you'll get the most value pulling from. SFL and Wonderland'll be worth a pull or two, but the overall card quality - not counting WD's neutral synergy cards - isn't as strong (plus being older). So yeah, go for DBNE and CG.