Krang is rumored to be inthe cartoon this time as an utrom criminal, Bebop and Rocksteady are also rumored but this time teamed up with Tokka and Rahzor which means that Tokka and Rahzar might be series regulars. and the channel six news crew may be back such as Irma Vernon and Burne thompson. granted that this info was on wikipedia and there just rumors.its also unknown if Karai is gonna be in ti and their have also been people wanting to see venus de milo again.
so far characters that are unknown to appear are
the rat king
fugitoid i already mention Karai
i would hope usagi appears but that might not be possible since i think stan sakai would not use him unless peter laired is involved peter sold tmnt in the first place.
and of couse the ones to appear are of course the turtles splinter april casey and Shredder at least i hope casey and april appear.