Honestly, the problem with discovery is not that there are not enough posts in a single timeline. Merging local and global feeds makes discoverability worse on Lemmy and kbin, not better, because the timelines display posts, while the space is organized by communities. This means that smaller or niche communities just drown seas of posts from large or highly active ones.
If you want a real "exploration" timeline, you need one that limits the number of posts from any given community. And that still seems like it's well served by local/global splits, because the website you join should be meaningful.
We do not need, nor should we want, a network of "dumb terminal" Fediverse sites. We should be aiming for the local stream to be the big selling point for any given instance, with the ability to interact with remote communities being a value-add. A merged timeline kills local identity, and tells users that their hosting website is a 2nd class citizen in the Fediverse.