Requirements for 6.2 garrison rare missions to pop:
- 3x followers with 675 item level
- 7x followers with 645 item level
- 675 item level on character (no need to equip the gear, you can just have it in your inventory)
- a minimum of 15 active level 100 followers
Those missions have 1 week cd; this means you'll get 1x rare 6.2 mission every 7 days per character, assuming you meet the criteria.
Now, you have just to cross your fingers and hope to get a "good" one (you can obtain a mission more than once after completing it the first time).
Garrison Timer Mobile App
New Toys in Patch 6.2