People, I'm afraid you are selling the bear's skin before having killed the beast. I'm conscious that this series took many things from the original version of the TMNT at the Mirage Comics, like Splinter finding the corpse of the Rat King. I read more information about this version, and I realized something big. The good news is that Splinter may still be alive, and the bad news is that Shredder might be alive too. It doesn't have too much sense to kill both the turtles' master and nemesis is this is not the last season. Let me explain:
In the Mirage Comics, Splinter died, but Donatello took samples of his DNA in hopes of restoring him to life some day. However, he eventually discovered that the Splinter they buried was not the same one that raised him. Apparently, at his death, a group of mysterious being known as "the Pantheon" took Splinter to offer him a place in their group, who commanded several kinds of animals. Splinter would have become Lord of the Rats, but he refused. Eventually, the Turtles discovered that Splinter was actually alive, becoming the Daymio of the Battle Nexus. So, if the Battle Nexus is going to appear, then there's a chance Splinter will appear here.
Sadly, Shredder could be alive too. Do you remember the Shredder's clones from Season 3? Who says Shredder didn't cloned himself in a more stable body to resurrect so anything happen to his corporea form? In fact, in the Mirage comics, Shredder was cloned after the Turtles killed him. Also, the "Mystic Foot" (the ones that we saw in the 2003's series as elementals) took a role too in the Shredder's resurrection. So, there's a high chance we haven't seen the last of him.