Platforms hosting original pairings like John Doe x Jane Doe come down to a few key places, but your approach matters more than the site name. You’re essentially building a fandom from scratch, so you need spaces that welcome original characters and original worlds alongside established fandoms. I’ve had the best luck on Archive of Our Own because you can tag it as 'Original Work' and still use relationship tags—readers there who filter by specific dynamic tags sometimes stumble onto original stuff and give it a shot. Wattpad is another obvious one, but the audience skews younger and is there more for the tropes than the fandom; if your John/Jane story fits a popular trope like enemies-to-lovers or fake dating, it might get traction there.
Don’t overlook Quotev or even Tumblr for serializing snippets and building a following through tags. The trick is to market it not as fanfiction of nothing, but as an original story with a ship at its heart. I’ve seen a few such stories even cross over to Kindle Vella or Radish later, which is wild. Honestly, half my reads on AO3 for an original pairing came from people who just searched the ‘Fluff’ and ‘Slow Burn’ tags, not a specific fandom.
Ending on a personal note, it’s a tougher slog for engagement since there’s no built-in fanbase, but the readers you do get are often super dedicated because they’re there purely for your characters.