I haven't followed all the discussion on I-35, but has there ever been a proposal for a complete over-under design where one direction (ex. northbound) is on top and the other direction (ex. southbound) is on bottom? This seems like an efficient setup that offers space for lanes and not having to widen the footprint. Example being the San Francisco Oakland bridge:
With the current discussions, it may not be ideally lined up with desires to connect East and West Austin. Personally, I feel like that is an perfect scenario that just may not be 100% feasible. Historically it has racial ties to segregation, but with gentrification the east/west areas of downtown don't exactly have the same vibe of 20+ years ago.