I want a sheep that is secretly an Imp, oh wait! lol no but some random and half thought out ideas that I'd rather enjoy;

-sandworm in desert that swallows you if you run too fast over a certain area with a chocobo that is off the main road. could take you to some underground cave zone or crazy boss fight
-areas and barriers that only open up during certain times of the day/month/year
-unique enemies that only come out at night and when its raining (dunno why I was so specific with that, anything similiar will do!)
-entrances to some caves are actually much steeper than they appear! and end up being one way trips
-I want do accidentally drop my weapon once in a while if I am spamming way too many abilities during a long fight. not really a secret but I think it'd be funny and random
-after killing like one-hundred of the same mob in one sitting, a super powered version of one will pop to teach you a lesson
-if you take on a very specific set of leves in one reset you will be approached by an npc when they are all cleared and given a secret bonus leve
-(this one is kind of from XI) falling/sliding down a slipper icy cliff a certain way gives you access to different areas of the mountain and battlegrounds inside them

uhh I guess I could keep going but I think that is enough for now lol but yes; secret=good