So as per @Kierunkowy74's reply to me, limiting (basically what I described) is a feature on Mastodon already. It basically just sets things to follower-only mode on a per-instance basis. I'm not sure how well that would translate to the threadiverse, but I do think some level of opt-in integration would be best.
To go on a slight tangent: I've never used Imgur as anything other than a image hosting site, but I'm aware it has people that use it as a social network in its own right. Whenever I've hosted anything on Imgur in the past - even images that don't need any context - I've noticed it always ended up downvoted and sometimes with some negative comments, while the reception on reddit was generally far better. It doesn't bother me - like I said, I just used it as an image host - but it's clear Imgur has its own culture. Threads could be the same, and trying to merge its culture with ours could prove difficult.
I don't know what full-on federation with Threads would look like, but federating vote counts could definitely lead to Threads culture overwhelming threadiverse culture. But I assume that's also something that can be done on a per-instance basis; I know kbin (which I use) already doesn't federate downvotes from other instances, for example.
I'm not sure I have a fully-formed opinion on it all yet, unfortunately. I don't like the idea of cutting Threads off completely unless they do something to earn defederation. I think finding a way to smoothly federate with Threads could give the fediverse a boost in users that could be significant for more niche communities that haven't managed to find a large enough audience yet (because yes, I'm still missing some of the smaller communities from reddit). But I do also think there are very valid concerns about both the long-term and immediate impacts Threads could have on the fediverse.