The Texas Department of Transportation has proposed adding lanes to the interstate, some with tolls, and moving the double-decker highway below ground level — drastic changes for a road that has been a major feature of downtown Austin since the early 1960s. The city, meanwhile, plans to “cap and stitch” the completed route with a series of bridges and platforms over I-35. What should be built on that new downtown real estate, and how should it be used to meet community goals? Some leading architects weigh in.