The Utrom Shredder is the one who killed Karai's parents.Karai's parents were Guardians, and Shredder killed her parents so that they could not interfere, he wasn't aware that Karai was their daughter, but he instinctively knew something about her, so he decided to take advantage of this familiarity to raise her as his Dragon.
Ch'rell was an orphan.Having fallen through the cracks in the Utrom system and forced to make his way by hook or by crook, he now has a sympathetic eye for people with similar histories, i.e., Karai and Hun.
There's significance to the way Splinter wears his robe.It might be a yukata, but it's hard to tell. He wears it right over left, which is normally used to dress a corpse for burial. This could be his own form of mourning for the murder of Hamato Yoshi. Now that Ch'rell has been Killed Off for Real, if Turtles Forever is canon, he may switch it to the standard left over right for a Genius Bonus in future episodes, if any.
The Eldritch Abomination is Cthulhu.It looks exactly like him in "The Darkness Within".
Bishop really hates the President.Think about it: while he's being chewed out by him, he gritted his teeth. It means that Bishop hates the President, and that's the reason why he uses the aliens to kidnap him and almost got him dissected in "Aliens Among Us". The part where Bishop having saved him is to hide the fact that he really hates him.
Bishop is one of the Wesker children.If you think about it, he has superhuman strength, speed, and being enhanced. However, after being abducted by aliens, it is assumed that he wants nothing to do with Spencer, his creator, and this is why he changed his last name from Wesker to Bishop.
The Utroms are responsible for Ch'rell's behaviorLets take a minute: why is Ch'rell so evil? why does he focus his efforts to the Utrom, something tells me that the utroms did something to either let him down or did something to him directly.
This series takes place in the same universe (metaphorically speaking) as Ratchet & ClankThere are a lot of strange parallels between the two works, but the most amusing is the existence of dimension-hopping devices where you would least expect them. Say, a flashlight and a hat. note There are also similarities between the Transmat and Gravometric Warp Drive, and both series (initially) utilize the Nanomachines trope for healing purposes.
Casey Jones is related to Kevin Levin.They have similar looks and talents. Both lost their dads at a young age. I take them as cousins. Kevin's mother is Casey's father's sister.
Casey's mask is a family heirloom.Lots of hockey masks are customized why not. Maybe its one of the few things he has left from his father and fights crime wearing it as a way to pay his respect and avenge his father.
The Demon Shredder that appeared in Back to the Sewers is without a human host.Notice how his hands are black in Back to the Sewers? In the Lost Season his hands were black in his pure Yōkai/Demon form while in his Hanyō (half Yōkai/Demon) form they are white. How he resurrected in pure Yōkai/Demon form is unknown though it may be revealed in a later TMNT series.