I feel like a domain is a nice way to link identities, with a small nominal fee being a nice deterrant to botting. Not the most user-friendly for those not tech savvy, but third-party services could help with setting up such sites.

Make a page on your domain with rel=me links to your social media profiles, have the social media sites link back to your site with a verified symbol next to the link when it scans and validates the rel=me link.

This puts you in control of your verification instead of federating it to a service like Keybase or Keyoxide.