WARNING!
This theory will contain spoilers from the episode Annihilation Earth, based on the book Mutants in Space!. Also this theory has already been posted by me on Instagram and Facebook, so this is nothing new for me.
So at the end of the Season 4 finale (after the death of Master Splinter), the Ninja Turtles, April O' Neil, and Casey Jones are rescued by Fugitoid and travel into time 6 months before the invasion. He explains to them about the Heart of Darkness being seperated by The Utroms and being hidden around the universe. So the story arc for Season 4 (before the anti-mutant campaign) will be searching for the three pieces of the Black Hole Generator before the Triceratons.
But the question is where these three pieces are located at. Well the following is the possible explaination as to where they are.
So I theorize that the location of the three pieces to the Heart of Darkness are located near Mona Lisa, Armaggon, and Lord Dregg. I do have some clues to back up this theory, so here I go. Buy first off, why would the Turtles meet these THREE characters if they are looking for the THREE pieces? Like there must be an explaination as to why and how they will meet these three.
Mona Lisa can be part of this theory because Raphael will have a crush on her. But what else? Ciro Nieli commented on Instagram that Zog the Triceraton will date Mona Lisa. But what does this have to do with the theory? Well the Triceratons are trying to get the three pieces. So maybe Zog is trying to lie to Mona Lisa on loving her, just to get the piece. Maybe there might be a fight between Raph and Zog to get the piece and Mona's love. But unlike Zog, Raph trully loves Mona Lisa.
As for Armaggon, there is barely any information on him. But there is a rumor that Donatello will have to rescue April from Armaggon. Since it is a rumor, I am not sure of it being true but I have something to back it up with. If this happens, then maybe Armaggon kidnaps April just to get the piece back, similar to Rocksteady wanting to trade Michelangelo for the Retromutagen in The Pig and The Rhino. So maybe Donnie rescues April because of this.
Lord Vringath Dregg is the last character. Here I have a lot of explaination. So according to his bio on his toy, it says that the Ninja Turtles fight Dregg because they destroyed his Robugs. But it can be that there is a reason as to why this fight happens. So maybe the Turtles are getting a piece of the weapon and then the Robugs find out, so they alert Lord Dregg. But the Ninja Turtles destroy them. But why would Dregg want to fight just for that? Well maybe the piece must of been in his base and hr found it important.
So this was my theory. Comment below. Do ypu agree or disagree? Why? Also comment if you want to see more of my theories.