What about Tumblr? Commonly when people ask to lose the 140-character limit I try and understand what people are trying to accomplish and how that is any different from what Tumblr has now.
After spending some time on tumblr, I've noticed the 140-character limit is great from a reader point of view. What it ensures (like you are alluding to) is that one user doesn't dominate my feed/attention with one really long post. I don't need curation on Twitter, because the time it takes to read 100 twitter posts is constant. It's harder to get trapped in an platform-imposed filter bubble on Twitter this way too because there's less pressure on the platform to algorithmically rearrange your newsfeed as well.
If twitter wants to survive they need to get content onto their platform which means loose the 140 character.
In some ways and ironically, Medium could be a kind of replacement for twitter if they found a nicer balance between long and short posts.