Several scenarios I can imagine.
A darker relationship between Belle and Beast, albeit toxic as hell and Belle snaps. They're both psychopaths and enjoy causing misery.
Or Belle became cursed by that Enchantress that made Adam the Beast. And she became wilder, more primal, savage even.
Or she became a Vampire idk lol it's funny
After the Beast dies from his wounds, Belle tells him she loves him, but it is too late, the last petal falls and the spell can no longer be broken. The servants lose all of their humanity and become objects fully. With Beast dead and her village no longer safe to return, Belle and Maurice inhabit the castle. Over time, Belle becomes a cold-hearted queen who learns black magic. She re-enchants the objects with dark spirits as her minions, to do her bidding. She brings the Beast back from the dead to get her revenge on the village that shunned her. She also brings Gaston back to torture him for eternity.
Beauty... has now become the beast!
"Even a man who is pure
And says his prayers by night
May become a wolf when the wolfbane blooms,
And the yellow moon is bright..."