Try using Cloudflare and having a wildcard subdomain and then letting others CNAME to it. You cannot do this on Cloudflare without purchasing an Enterprise off

Try using Cloudflare and having a wildcard subdomain and then letting others CNAME to it. You cannot do this on Cloudflare without purchasing an Enterprise offering with SaaS features. It's a very basic thing, allow CNAME to your subdomains... but nope. So if you want to do this, you cannot use their DNS or Registrar.