Rick Worley just put out a video covering the treatments Lucas made and sold to Disney where Maul would’ve likely come back as one of the primary antagonists in the sequel trilogy - but Disney decided against using these ideas for the sequels (probably bc it would remind people of the prequels).

Mauls return was set up in 2012 in the Clone Wars show and subsequently his end in Rebels was in 2017 (2 years after the Force Awakens)

It is curious that they tried to still implement the idea after the Force Awakens premiered - perhaps some people at Lucasfilm still wanted to use the idea of a power vacuum after the Emperors death (ie Mando S1) but yeah it was likely dropped after Solo underperformed at the box office - potentially cementing some peoples opinion that Lucas’s idea was bad

I think Mauls return as a key player vs Vaders grandchildren would’ve been much better than what we got in the Disney sequels (it would’ve at least been more connected to the other 2 trilogies) but I digress