Shredder is more important then Tatsu. So how can you before one in a new movie and not the other. Tatsu is Shredders right hand man without Shredder Tatsu is useless
In the second movie, when Shredder fell Tatsu was going to take over as the leader, which is what would happen if Shredder is not to come back.
When Eastman and Laird created the Turtles, they did not make Shredder thinking "arch villain for the rest of the stories" but 'oh what could we have them fight and kill?' In Mirage Issue 1, the Turtles don't just fight Shredder they kill him. The only thing that came back was a clone and they killed that as well. So it was never Eastman and Laird's idea for Shredder to be in every media all the time.
So yes, I can go through the movie without Shredder just like we have for the last two movies to come out. But if the foot is to come back, at least they can keep some sort of thing that we all know is part of it. Tatsu. True he was a right hand man, he was right there for Shredder and if anything as a new leader he would be working to make the Foot stronger, than go after the revenge.
In the second movie when Shredder came back, Tatsu was all up for rebuilding the Foot, as Shredder line was "There's only one thing next, Revenge"... he was so focus on revenge and Tatsu being 100% loyal to him followed his every word, even though Tatsu was right on what should of been next. I would personally enjoy to see where he would take the Foot as the new leader. Though now with the 4th movie we also have Karai in the picture. It'd be interesting to see if a writer can really mix the two.