Mordecai's romance episodes were fine to enjoyable, only Do or Diaper, 1000th Chopper Flight Party, Not Great Double Date, and Dumped at the Altar are trash (I love the You Might Think montage in DoD though)
Rigby was better as a jerk, and shouldn't have ended up with Eileen.
Benson should've went back to being in a band, performing with Mordecai and Rigby in private for park anniversaries or something, Mordecai keeping his main art gig and Rigby becoming a video game maker.
To keep JG's student film lore of Pops being from Lolliland, keep all the Anti-Pops shit but pad it out more (this is more CN's fault), in the final few episodes, instead of DVD/Bluray's Format Wars III arc, have them cross paths with Klorgbane because they went into his path's orbit. Due to Anti-Pops they have to leave Klorgbane and have Pops fight at Lolliland. Just like with streaming, Klorgbane shows up and aids Anti-Pops in attempting to kill the park guys. Skips gets the upperhand and uppercuts Klorgbane again, sending him flying. Just when Anti-Pops and Pops are about to collide, he gets between them and is fucking obliterated. The final 11 minutes is of the team aiding Pops in defeating Anti. Pops is severely injured, but before he's killed everyone just starts beating the shit out of Anti, it doesn't work and he fights them off, they realize the only thing that can kill an immortal is another immortal or some shit and Skips has to die with Anti akin to how Pops died with him.
After that ass of a montage, the spirits of Mona and Skips are together.
