I definitely see Shredder being already introduced in Season 1, but he might be more of an "Emperor Paplatine" character in he's mostly in the shadows for a lot of it till they're ready for a reveal.
Now I think other familiar characters won't appear till Season 2 or later. I do think eventually we'll see the usual Karai, Baxter, Rat King, Leatherhead, and some form of Krang or the Utroms, but not till the second season. Unless Baxter is April's father which I really like the idea of.
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