Eastman refers to Shredder as "Karai's father" in Ult. collection 5

[Rat_King]

Originally Posted by Andrew NDB

Might have been an early idea that Kevin bounced off of Peter that they collectively dismissed... and it's hard to say exactly how much Kevin was involved in "City at War" directly, creatively.

Yeah, it's been mentioned several times that Kevin and Peter didn't agree on much. It's very possible that it was what Kevin wanted but since Peter more heavily oversaw the arc it's what he wanted and what they went with.