The Stretch Goals Mission consists of seven objectives, which will net you a tasty 52,000 XP bonus each. Interestingly, this time around if you complete all seven of the challenges, you will be able to unlock “Prestige Mission Objectives”. However, this unfortunately means no new loading screen.
These are the Stretch Goals objectives:
Deal damage to opponents (5000)
Achieve Shotgun Specialist or AR Specialist by dealing damage with Shotguns or Assault Rifles (10)
Achieve Pistol Specialist or SMG Specialist by dealing damage with Pistols or SMGs (10)
Achieve Sniper Specialist or Explosive Specialist by dealing damage with Snipers or Explosives (10)
Catch weapons from fishing spots (10)
Eliminate opponents (25)
Outlast opponents in Solos, Duos, or Squads (2000)
Although you can't see the Prestige Mission Objectives in-game until you have completed the seven challenges above. Fortnite leaker FireMonkey may have you covered though, check these out.
If these are correct then they are going take some real grinding to complete, but earn you an insane amount of XP. Time to get to work.