I'd suggest doing all the WiFi side quests first, and then do what Elpizo said:

Quote Originally Posted by Elpizo View Post
Start training to beat the Iron Giant if you haven't already, get Onion Knight, get every job level for every character to lv99 for every Ultimate Equipment, get max HP and get every character to lv99. Should keep you busy for a while if you haven't done this already.
In addition, if you haven't got summons like Odin, Leviathan or Bahamut yet, you should try to get them.