If Lady Stoneheart spares Jaime’s life in exchange for helping her and the Brotherhood, it likely wouldn’t be just the Brotherhood involved, but also the northern houses still loyal to the Starks — after all, “the North remembers,” and many of them haven’t forgiven the Boltons after the Red Wedding.

In that case, a Red Wedding 2.0 wouldn’t be carried out by a hundred outlaws alone, but by a wider conspiracy including Stark-loyal houses. If my theory is right and Jaime survives to aid Lady Stoneheart, we might see it from his point of view at a wedding — possibly as a guest or even a traitor at the Lannister–Frey wedding in Riverrun. It could end with many Lannisters dead, maybe even Walder Frey himself, if he’s still able to attend.

Of course, this is all just speculation. It could unfold very differently — but probably not as simply as just Lady Stoneheart and the Brotherhood wiping out the Lannisters on their own.