I'm already having fun but of course this needs up keeping to be sustained. So I'll start off with more end-game content. Ifrit and Moogle plus the occasional Darkhold is still fun, but I'm definitely looking forward to the new dungeons coming up and the following Primal battles.
Fixes to the lag issues will also greatly raise the fun factor. Getting really tired of rebooting to fix timers or getting system messages telling me my skills aren't ready when they're showing me otherwise.
PvP. Although with the current combo positioning system, this might be a bit tricky. I see the mages dominating PvP if the battle system remains the way it is.
I'm also curious to see what they do with Battle Regimen. I liked the old idea of how when used, it would inflict certain debuffs on the mob or buff the PT. I just think the execution of the system was sluggish and hard to understand. I think it would be a good idea to use it in the same way solo comboing works. Keep the 1 > 3 > 5 rules for comboing but allow other PT members to fill in the links of the chain, and have specific jobs who end the combo inflict specific debuffs/buffs. They could make it fairly easy and yet complicated by having different combinations of jobs doing different things (i.e GLA > LNC > THM would do something different than MRD > ARC > THM but the buff/debuff will still be relative to THM or in essence, offensive magic), or just making it easy so that which ever job class ends the combo does the same debuff/buff no matter what the combination is. It could be set up just like comboing so when someone uses a WS, certain WS/Spells in the PTs AB would light up and they can just easily use the corresponding WS/Spell to link the combo in the 10 or so seconds you normally have when you solo. Of course it doesn't have to be like this, I just like the idea of having PT members being more interactive with each other. Like SCing in FFXI, it's a nice way to add more teamwork to the group and can even be very effective.
I just want more epic battles to be honest, and as many of them as possible. Just please don't add any 2 hour long Dynamis-esque instances that just drag on and test your patience.