Mutant Ninja Turtles Gaiden/WMG

Donatello plans to recreate his existence.
With everything that has been said and alluded, especially towards the end of chapter six, it would seem as if Donnie is trying to make turtle clones from Renoir's cell tissues. He says that he wants to get the reality that Leonardo stole from him from back then. He may even go as far as to implant his old memories into a new body for himself; this could also be true because it would explain how Renoir killed his human body but retained the memories of his human identity, Augustus Amherst.

Donatello will eventually fight Leonardo
While this seems to be another wild guess, it just may happen considering that. Before Renoir came on the scene, Donnie has been going through ninjutsu training in secret just enough to match Leonardo or even surpass when they finally confront each other. Given Donatello's intelligence he may have even found a way to enhance his ninja skills via science in order to best Leonardo in combat.

Donatello plans to use Renoir's mutation to revive Splinter
Think about it. He's absolutely desperate for a real sample of the kid's blood after noting, primarily, that it increases his healing beyond even the standard mutation. However, he's desperate to get it in a controlled setting; it's obviously useless to him if he doesn't get enough to experiment on, and in a way that he can experiment with it.
 * This isn't so much a WMG as it is a hasn't been confirmed yet. If he wants Renoir's cells for any other reason it'll be a shock to us all.
 * Gotta double down that this isn't a WMG. This is all but confirmed with dialog. The wild mass guess is what is the curse doing to his mind? Does it really thrive on pure unadulterated irrational hatred? I ask because Renoir is a new mutant with no apparently explanation as to where he came from and why his mutation has better healing and perhaps other traits that set him apart from the first four. If Donny politely walked up and said: Renoir I'd like to know how where this turtle body came from, how you got stuck in there and why you heal so fast and I think studying you're blood might yeild results I'm pretty sure Renoir would donate blood in a heart beat and I don't think any of the other turtles would suspect anything at all.

Donatello plans to use Renoir's mutation to help him kill Leonardo
Same main reasoning as above, only instead of reviving Splinter, he'd use it to make his new fighting style unstoppable. He's still a ninja, after all; being a ninja's all about getting the edge on your target and using it as hard as you can.
 * Unlikely. He clearly doesn't know how to make cells heal faster and nothing we've seen of Renoir makes me believe that his healing would be of much use in combat. I suppose it might give him the edge that he can fight looking for a wound not a kill take a shot to hurt Leo then fight again the next day and rinse, wash, repeat but really bad plan over all unless he can advance Renoir's healing by at least double.

Augustus really is dead, and the turtle absorbed his memories and personality
The hypothesis is that the turtle that attacked and ate Augustus either got at his brain and took in Augustus's mind that way, or that the turtle was somehow affected by the sword's curse and absorbed it magically, depending on who you ask. It could go either way, though, as...
 * wouldn't that mean that he basically IS Augustus anyway?
 * no, it means he's Augustus with an angst-worthy tomato surprise.

The feral mutant turtle was either a remnant of the cursed sword on the Hamato family OR was made by the shadowy figure in the prologue.
...on the one hand, the curse is explicitly stated to affect the family later in the story, which it does by being someone Raphael takes in and feels guilty about occasionally, and on the other, we've got a mad scientist taking mutant blood and being very pleased with himself in the prologue. Could be one, could be the other, could be...

The feral mutant turtle was made by the shadowy figure in the prologue AND ALSO was affected by the curse, by technically being part of the Hamato family.
If the mutant turtle was a clone, it was directly part of the Hamato family by being, technically, a twin brother of sorts. If it was made by mutation extracted from the blood, it was still made from the blood of the family and could be considered, technically, a son. Whew, that's a lot of guessing.

Tetsu is .
As of chapter 21, all evidence seems to point in this direction:. So, it would a real twist if we will discover it's not the case.