1) Complete the 2.55 story (need to access 3.0 main story).
2) Unlock all fights related to your daily roulettes (you'll be using them to level your main).
3) Collect Grand Company Seals (you should be able to rank up within the GCs in Heavensward).
4) Collect Retainer Ventures (Retainers will be able to go from 50 to 60, so you'll want to have stuff set aside to level them while you're ranking up in your GC).
5) Collect hunt seals (even if it's just your weekly and the occasional A rank, they might come in handy).
6) Level your Chocobo to Rank 10 (that little guy is probably going to be doing FATEs with you).
7) Level any side classes (this includes crafting and gathering if you're into that).
8) Gear any side classes (I have all my DoM/DoW classes sitting at i120 as I think that's a good start for 3.0).
9) If you're interested in the new jobs, get Tome + Sol tickets + Sands so they can at least have an i110 weapon right at 50 (they're not getting i80-i135 relic weapons).
10) Save your Gil!
11) Stock up on some cheap NPC food. That 3% EXP bonus is going to be your BFF.
12) If you have nothing to spend Sol on, buy and sell the crafting mats for a little bit of extra income.
13) Make sure you're capping on Poe weekly. You only have 2,700 more Poetics (including this weeks) to obtain before Heavensward comes out.

I think that's all of what I'm doing right now (not including working on EX Ponies and SCoB Savage with my static). I like to be prepared for things and I feel like I'm nearly ready for this expansion now. :3