Hello LJDawson, let me start with saying how happy I am to have Sync back :)
The one issue I have with Lemmy is duplicated posts between different communities (see screenshot). Posts usually have same title, author and link, yet differ in community/server they are posted to. It would be great to be able to make the appear as one item on the feed with some note below that links to both instances' posts, or something like that basically to make them not appear twice or thrice one under another.
There are a few ways to handle it, but yes it would either need to be done client side or via a server proxy.
I am actually toying around with something (that I may or may not release publicly, depends on how far I get before I lose interest) to help app developers do stuff like this without heavy processing on the client.