I think among other issues would be the Gmail-ification and iMessage-ification of the fediverse. What I mean by that is open standards like email are dominated today by many people using Gmail accounts as it is popular, “free”, and comes with a ton of features. Then google started “walling off their garden” by adding features that only work between gmail accounts. Similarly, apple also took the open standard SMS and started adding on features only available between other iPhones.
What we might see is some of the coolest features the fediverse has ever seen, but it will come at the cost of most users ignoring or dealing less with "irrelevant" things not on meta ran instances.
Hope we can resist such a change, but that is what I am concerned about.
Even though email is supposedly "open", and federated, is no longer is really the case. Big services like Gmail are suspicious of non-big-name servers, and often flag email coming from them as spam.
About a year ago I came across an article from a guy who'd been running his own email server since the 90s, and finally gave up. I couldn't find that article in my quick search, but I did find this:
"I run my own email server. It's no longer a good idea, because the anti-spam arms race makes delivery from small independent servers very difficult, even when you keep yourself off the block lists, so it's a continuous struggle. Would switch, but I have too many domains/addresses"
This is very true, I have hosted my own email before and if you are doing it yourself and not going through a big player like google to host it then your stuff sometimes gets treated as suspect by filters. Used to beg people with Gmail accounts to flag my emails as "not spam" whenever it showed up in the spam folder.