Read an Interview with Ion, where he clearly stated they added AOTC mount and mythic appearances simply because they felt like it and he does not even have any specific reason for that.
After that you will stop looking for any logic or reasons behind some fomo returning and some not 
- if you like fomo, better get it because there is no guarantee it ever returns, this way Blizzard keeps you unsure and makes you play current content
- if some fomo returns, you will never know in advance which it is and Blizzard will mostly use it as a carrot on a stick to temporarily boost the subs - there is no logic behind that, they will simply choose some fomo and use it exactly like Ion said - because they will feel like it at the moment and they will devalue any fomo you can think of in desperate need of subs and revenue, be warned
- or ignore fomo and no stress at all