The topic of federation has come up earlier here here and here. I’ve now made a presentation on this. Longer than I wanted, sorry. (Not enough time to do something shorter 
) Here is the playback, which includes some links to this forum and some other resources Playback. The slide deck is here: Nextcloud
Please do give this a viewing, and please do come into this thread with responses. By all means start parallel topics. This might be a good way to develop develop a sub-themes (like plural language, or diverse toolstacks, or single sign-on, or regional/municipal focus - whatever) , but please link back to here?
Update - for clarity here, a separate thread has been opened on Federating.