Do you think they would try to kill off another character?

[Splinter the boss]

Originally Posted by IndigoErth

If anyone... Shredder, once Nick is done with him. (Heck, they already killed the 'tongue' clone of him on that one mutant monster...)

Though Karai is a lot darker option. However, as long as they return to her story I think it's more likely that they'll probably eventually cure her (or at the very least cure her to the extent that she gets her brains back together, can control her mutation and be herself most of the time). And therein either be back with her real father or, since he may not want her having to live where they do, return to Japan to live with extended family for the remainder of her youth and leave the series.

Spoiler:

Then make a second series as a continuation where all the characters are older and Karai moves back to NYC as a young adult.

It would be nice that way. It would be heartwarming when they would reunite in the second series and would fight crime as a team.