In "The Clone Wars" Series 5, Episode 16 (The Lawless), Darth Sidious defeats Darth Maul in battle and tells him he has "other plans" for him.
What were these plans?
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Darth Maul's story continues in Son of Dathomir comics from 2014, a follow-up to unused Clone Wars cartoon storyline from season six (the episodes weren't finished before the season was cancelled halfway). Sidious' plan was to
to draw out Mother Talzin, leader of the Nightsisters, so she can be destroyed as a potential threat to the eventual Sith rule of the galaxy.
Talzin was a Dathomir witch, mother of Darth Maul. Her being poweful, and the fact that she and Sidious had a little bit of history, he wanted her dead.
The plan involved letting Maul escape and have him lead them to Talzin.
The story is part of The Clone Wars Legacy line.
Unfortunately it was never disclosed. But I would guess that he wanted him as some sort of dark agent or maybe as a backup in case his plan for Anakin Skywalker fell through.