A helpful quest tracker to help 100% all the pesky optionals and quest ranks.
Optional Checklist
Doing these objectives unlock optional quests
Finding a monster, low/high/master rank all count as separate.
Capturing monsters, low/high/master rank all count as separate.
Talking to Villagers with a (!) in Astera, Research Base, or Seliana.
Completing previous Optional quests.
Randomly available Rare encounters that last for 2 quests and disappear.
Fully researching monsters (all tabs in the monster manual).
Hunting a certain number of different tempered monsters (quests/investigations with more than one tempered targets only register the first listed).
Your current progress level for Assigned Quests.
Special Arena: Quests are unlocked by Capturing the monster first. Once completed, they appear randomly on your Quest Board. Keep in mind that if you capture a Low Rank Barroth, it will only unlock the Low rank Special Arena.
(!) => Those are Villagers' Requests
Rare encounters : Zorah Magdaros, Xeno'Jiiva, and Shara Ishvalda optionals - Be sure to check your quest board as these only last for 2 quests total. You'll hear music in Astera/Seliana change and a notification sometimes.