I still think the Expo/Arden area is a great place for a "second downtown" of high-rise projects--something like Century City in Los Angeles. Consolidate all those parking lots into one parking structure, put up a couple towers, add residential mid-rise/high rise nearer the freeway (and thus nearer to the dramatic views of the river) and a fixed-rail transit loop up Expo and down Arden to link it to Light Rail and downtown. Think of it a bit like Century City.
To contribute, here's a drawing from an early 20th century plan for a civic center around Plaza Park (now Cesar Chavez Plaza) from 1916 that never got built:
