Lets keep talking about Clybourn and Elston
A subway line running under the entirety of Clybourn has been long justified. Also feel the bike lanes should be improved upon and extended northward towards Belmont.
I think the natural center of gravity for the north branch is probably North & Clybourn. There's already a ton of foot traffic and it's largely box stores & surface/garage parking. Could easily be more than it is today.
Fullerton & Clybourn could also evolve into something similar / secondary node with some major infrastructure improvements (ideally road diets for both Ashland and Fullerton).
I think Elston is one of the few streets in the city that could, not without significant headache, go mostly car-free entirely south of Cortland. 90/94, which is the reason Elston sucks so much, is literally right there and traffic is already fairly light in my experience.
It would take a bit of time for its use to change into something more destination/entertainment oriented, but it could happen with the right plan in place.