Anyone that's seen Season 3 has known the series was putting nicely wrapped bows on everything
-Lena and Magica got an ending chapter
-GoldiexScrooge got an ending chapter
-Penumbra got an ending chapter
-Gyro and Fenton's boss/employee relationship got an ending chapter
-Owlson and Glomgold have winded down
-Donald met Daisy, and Della's back home (as per the original two season outline)
-All the Darkwing and Launchpad build-up had its ending chapter
-The Greek Gods, BOYD and Doofus, Gladstone, Caballeros, Castle McDuck family all have had serviceable ending cameos too
There are only THREE unresolved plotlines heading into the final 6 episodes in early 2021(75 half hours so stop spreading the 65 episode hysteria):
-The final confrontation and disbanding of FOWL that will get to be Huey's big moment
-Revealing the secret Beakley has kept from Webby
-Gizmoduck and Gandra Dee's romance (and maybe paying off some of that depth they were giving Mark Beaks, but not a necessity)
I don't doubt that 25 more adventures could be pulled from the comic for a Season 4, but you have to remember from Disney's perspective this already was the bonus season that many fans are going to petition for
The biggest mystery is "who is the dad" though, even if the answer is inevitably "our real family is right here".