Mettaton's route has been added to the game! The epilogue / bonus story content will be added at a later date. This route was commissioend by @Rory on discord through Kofi.  Below is the walkthrough.

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• Make sure to keep following the general main walkthrough to grab all essential items, otherwise you'll get the neutral ending.
• Be on the friendship route when entering the City/Core area
• Be bold!
• At the end of the talk show (assuming you've been boldly flirting when able) follow the below choices:
• "The sun?"
• "But! He overcame it!"
• Order the Razzle Dazzle / Order the Legendary Hero 
• Ask all of the questions - when it comes to the idol if you say yes, then say "You want the experience" and if you say no then say "There's some things in life..."
• Don't try to fake it. :) He can tell.
• If you've succeeded on the date, he'll give you a pin & the ending screen will mention that he's thinking about you.