Sasuke is in possession of an Eternal Mangekyou, the first tiime in history since Madara Uchiha himself. This perfects his Susanoo, and I believe this is what was seen when he first demonstrated its overwhelming form and power with Amaterasu Orb against the last bit of White Zetsus. I strongly think Kishi is suggesting that the form Sasuke’s Susanoo takes when synced with Naruto (who is in perfect control of Kyuubi’s chakra) is of the same level that Madara achieves with JUST the Rinnegan alone - which would make sense, that the Rikudou Sennin wielded a Susanoo of titanic proportions capable of competing with a Bijuu. So with this said - Sasuke’s final power-up may involve some other unique but essentially the same way to achieve such a God-like Susanoo on his own (please Kishi), or it may have already been shown as the combo with Naruto we’ve seen (borrrrriiiiiinng). But either way, his final power up will revolve around Susanoo - the ultimate technique of the Uchiha.