Alternative endings for alternative mutations ... and alternative combinations of mutations. For example: Big wings and big teeth allows him to fly into the sky and eat God. Small wings and fat head allows him to penetrate mother earths huge vagina.
When the worm grows amazingly big, let him fight blue whales, and then giant squid, and then Kraken, and then only Hellboy will be able to stop him.
If the unthinkable should happen to our dear Hellboy, then the worm could jump from planet to planet and eat the solar system.