I think Nostr will evolve to have a mix between a badge system and a DNS Text record where you can cross-auth/link accounts programmatically with fast/easy verification. Kind of like keybase.

It will likely end up being a mix of push and pull - where some services like twitter will never help verify your Nostr account, while some may support an easy way to link your profiles - again the key being fast and easy to verify from both sides, as each side could be deceptive.