Sanji already destroyed the RS and that event was a significant milestone with regards to his character progression so no, even if the RS does end up coming back in some shape or form, Sanji won't be using it. That said, the question of Sanji's potential powerups is a very interesting one, as out of the M3, it's not quite clear what route Oda intends to go with Sanji so I'm quite intrigued by what Oda is doing with him atm and looking forward to seeing what he dishes up going forward but I do think that it'd be tied to his fire in some shape or form. He already has the hottest flames in the series (i.e blue flames) so going forward, Oda could potentially go an Amaterasu type route with him or even enable him to utilize flames in different ways as well ^^.
Also I know some Sanji fans don't like the Sanji/Lunarian powerup route and there are also some Sanji haters that love to take the piss with this but I do think the whole Sanji/Lunarian ting was brought up for a reason on Wano, so I'm sure it'd be re-visited later, potentially tying into his next powerup. Again not really sure what that this will bring but it's interesting nonetheless.
Lastly with regards to Haki, I don't think Oda is going the same route he is with Luffy & Zoro with Sanji, I do expect his CoO to progress even further at some point in the series but I don't think Haki is gonna be the main part of his strength progression going forward (could be wrong tho). Regardless, even if I'm right, don't let the Haki fanatics delude you into Haki is the only way for a character to be strong. Unlike the other members of the M3 Sanji didn't even get a Haki boost on Wano and still put Queen in a spliff with Ifrit.