BB Pirates with Lunarian/Pacifista Upgrades make more sense than Warlords with such Upgrades, cuz it's not like They are deceased, we already have Mihawk & Mr. 1 still Active & Important, we still have Crocodile & Moria & Jinbe & Hancock, so imo Warlord Seraphim were only created to pave the way for BB Pirates Upgrade.
The Seraphim are Oda's cheap way of having the Warlords still fight for the WG, while each Warlord gets their own personal story as members of CG or of the Grand Fleet.
The BBPs as Seraphim would be cool,
visually, but power-wise, what would change? You'd be just sticking the Lunarian flame on them. That invincibility gimmick is already getting old after becoming the focus for the third arc in a row, having an entire crew spam it would be too repetitive. Plus, how would it even work in conjunction with certain DFs? Like, can Pizarro merge with a mountain and still be able to activate his flame to negate any attack? 'Cause in that case, your idea would make the BBPs even more overpowered than just giving them aCoC.
I don't see that happen. They could get a scientific upgrade by adding Green Blood to their current DFs, though, kinda getting each BBP two powers, with BB alone having three. That could be a route.
And I do wonder if an S-Kurohige is in store for a future reveal once BB attacks MJ. Or even... an S-Rocks? IMU using BB's father against him as a trump card? Garling might have salvaged his corpse before GV went down.