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 LeonardoWhile this seems to be another wild guess, it just may happen considering that Leonardo has returned to New York in chapter fifteen. 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 SplinterThink 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.
Donatello plans to use Renoir's mutation to help him kill LeonardoSame 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.
Augustus really is dead, and the turtle absorbed his memories and personalityThe 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...
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 not Davianna's father. As of chapter 21, all evidence seems to point in this direction: Sayako was so broken and desperate she seeked comfort even in the man who attempted to rape her; on his side, it's evident his inner turmoil was bigger than he let see (especially given his twin brother's death). The recipe to Sex for Solace seem perfect, the two would then part ways, and Sayako would later discover her pregnancy. So, it would a real twist if we will discover it's not the case.