After Vader's defeat at the hands of Obi Wan Kenobi on Mustafar, Vader's powers have severely diminished due to his somewhat severed connection to the force. Sidious went to Mustafar and saved Vader, but locked him in a suit that instilled much pain in Vader to enhance his anger and his connection to the dark side.

Why did Sidious save Vader? It is not the way of the Sith. 

I know there was always the thought that Sidious saved Vader because he figured he wouldn't find another apprentice that is even half as strong. Other people believe that his plan was to use essence transfer when Vader surpassed him in power. 

I am wondering if Sidious still feared Vader's power, even when Vader was weaker and crippled. 

When Maul was defeated by Obi Wan on Naboo, Sidious didn't even investigate to see if his apprentice survived. He just assumed that Maul was dead. Maul later came back into the picture and conquered the crime syndicate, potentially with the desire of overthrowing his old master.

If Sidious had left Vader to die on Mustafar, there is the possibility that Vader would have been saved by someone else, such as a Mustafarian. If Vader was saved by someone other than Sidious, it's possible that whatever suit would keep him alive would have been less painful on him. Maybe he could've been saved by a night sister, as Maul was, and "reconstructed." As such, maybe he would come back to the light, despite his desire to bring down the Emperor in vengeance. Had Vader returned to the light or even stayed in the dark, he might have found a different means of gaining power to overthrow the Dark Lord, just as Maul might have planned originally. 

All hypothetical, but I have to wonder if Sidious feared what Vader could become had he been left to die, but miraculously survived.

Do you think this fear could have existed at the back of his mind?