This sounds really stupid but.....

[The Fifth]

Or ... what if Mount Doom wasn't even an ACTIVE volcano?

And so Sam and Frodo climbed wearily up the craggy mountainside, scrambling over sharp rocks, stepping tiredly over charred surfaces, and soon making their way up to the top. Frodo stood at the edge, and took out the ring. He wanted to throw it away, get it over with ... and yet ... it was HIS. A maddening thought fluttered over his head, swooping into his mind. The ring was his. His own. He claimed it.

Gollum leapt in front of Frodo, shouting, 'IT IS OURSSSSS! PRECIOUSSSS!' and bit off the hobbit's finger, spitting it out and clasping onto the ring, pushing Frodo back and dancing happily. 'Precious! Preciousss!' Then he tripped over a small pebble, falling into the blackness, which happened to be a foot-long drop, so the creature was unharmed. He was IN the Cracks of Doom. Gollum peered about. Then, suddenly, the Eye of Sauron appeared before him in a mighty, fiery flash. He boomed, 'MY RING CAME TO BACK ME'

Then Erestor appeared. 'Excuse me ... but where did you come from, Sauron?' asked the elf.

'Where did YOU come from?!' boomed Sauron as he reclaimed his ring, his spirit binding together, and all his powers back. Erestor shuffled away as Sauron regained his body.

Meanwhile, Frodo and Sam were cuddling on top a rock, offering a lovely view of Mordor and its ashen sky.

Then, suddenly, Arwen rode in on Asfaloth, making humourous quacking noises and swinging her father's sword around. And then Arwen married Legolas. Then Earendil spanked Elrond for "being a bad boy". Then Eru lit some candles and danced around like a silly person.
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That is what have happened if Mt. Doom were not active ... yes, I know I'm not funny.