Can't disagree with any of your suggestions. Here are mine:
17th and California & 17th and Welton
Two prime parcels along 17th, convenient to light rail, in the heart of the business district. These empty lots are especially offensive since the 17th and Welton parcel isn't even surface parking, just a field of dirt. I only suggest these because this thread is "for fun" - I see no chance of either of these being developed in the next decade.
Federal Reserve
I throw up in my mouth every time I walk by this piece of shit. Another block that probably has no shot of redevelopment any time soon.
Block C
Full block of surface parking right across the street from Coors Field and in the heart of LoDo - just seems unnecessary. Put something on top of a parking podium and be done with it.
Colfax and Grant
Not so much in the heart of downtown, but a street wall surrounding the Capitol building only seems right.