If it really starts supporting ActivityPub, you could start seeing Threads posts in a fediverse feed that also includes posts from Mastodon, Reddit alternative Lemmy, and any other platform that runs on the protocol.
Please no. The last thing Lemmy needs is a tsunami of Facebook normies. Instant Eternal September.
I would argue meta is the only way mainstream content creator are willing to join fediverse, It won't be the worst thing to have more options, especially when feed curation in on our hand.
Be the same as Twitter/X and Reddit. Use the Microsoft Embrace-Extend-Extinguish approach. Get some nice third party clients using the API, realise you can't pump ads into them, block them and force people to use the official apps because everyone is now using the platform and can't go anywhere else.