Are we sure Leatherhead's back on Burnow? Feels like a waste of an arc to bring him to New York, he's much more interesting smack dab in the middle of everything (not that plenty doesn't happen on the island but you get my point).
As for Null, there needs to be something to look forward to after #100...
The Archie version was one of that run's biggest villains and she has even more going on here. More than a fitting successor to Shredder, Krang, Kitsune and Bishop.