I agree with you that Palpatine was planning on turning Anakin, but I think he was flexible on how that happened. Bear in mind there were clear costs to appearing to lose to Windu - his whole seduction hinged on Anakin believing that the dark side had more power so that he could use it to save Padme, and seeing Palpatine on the floor with no lightsaber and Mace Windu apparently triumphant is not the best way to convince Anakin that the dark side is more powerful. So, I think plan A is for Anakin to come in and find all the Jedi dead, so that Anakin believes the dark side is more powerful, plan B is for Anakin to see Palpatine's lightning prove too strong for Mace Windu, so that Anakin believes the dark side is more powerful, and only plan C is for Anakin to get involved himself. I don't deny that plan C was there from the beginning; I just deny that it was plan A. In my view, only when Palpatine couldn't defeat Mace Windu - either with a light saber or with the lightning - did he get Anakin directly involved. I know many disagree, but, as I said, I keep my own counsel on which interpretation is the best one.
Anyway, in more up-to-date matters, I'm pretty annoyed about Palpatine apparently being in episode 9. The whole point of the ending of Return of the Jedi, it seems to me, is that Anakin Skywalker eventually defeated and destroyed Darth Sidious so that Sidious thereafter did not exist in any form whatsoever. So, I always refused to read the expanded universe storylines with a revitalized Palpatine getting shot in the back by Leia. Perhaps there's a way for them to pull off some version of a Palpatine force ghost, but I'd prefer him simply to never be heard of again after Return of the Jedi. On the other hand, they need to ret-con force ghosts of Obi-Wan and Anakin into episodes 7 and 8 yesterday.
