A way to view groups of multiple communities in a single feed. Like a multicommunity. A workaround right now is to have multiple accounts, which is less than ideal.
A way to view content from multiple accounts in a single feed. Like a universal feed.
An easier way to have the same settings apply to multiple accounts.
Regarding #3: a way to copy/synchronize subscriptions between accounts.
I use a main account and a backup account on another instance in case the main instance is down, so I want those to have the same subscriptions. Manually synching them would be fine (maybe with options like copy everything, mirror the subscriptions, make sure B does not contain the same as A, etc.).
You can filter instances and communities. For clarity, you mean blocking all users from those instances from your feed/comments? Do you also mean blocking all users from a community from showing up? If so that seems harder.