No, you can't bring Sai back in a official manga. The point of the NorthStarCup arc and indeed of the time after Sai's disapearence is that Hikaru is forced to find Sai within himself, and to defend him where Sai cannot. If Sai where to come back he'd gbe able to do his own defending and he'd effectivly take the wind out of Hikaru's sails. So no, he can't come back officially.
Re: Sequel?
The point of the NorthStarCup arc and indeed of the time after Sai's disapearence is that Hikaru is forced to find Sai within himself, and to defend him where Sai cannot.
If Sai where to come back he'd gbe able to do his own defending and he'd effectivly take the wind out of Hikaru's sails.
So no, he can't come back officially.