If a central library were to be put there, a lot of new building would have to happen. From looking at Google sat image, there looks to be a good 1/2 city block, plus some irregular left over space to the west. I'd agree punching a few holes in the ceiling and bringing some light into the current building would be a good thing. I'd be thinking 360 IMAX for the current Dome if possible. Frankly, I'd like to see the city put a little money into a request for proposals - at the end of the day, it may not work out, but that would be the best way to find out. The art gallery would be good alternate choice in that case.
With an LRT station down the street, Shaw Millennium park, Mewata, a ton of residential, the river, and, no need to tear down the current library, it looks pretty appealing to me. I'm not fond of building in the east end as my first priority in a central library is it should be no more than one block walk from major transit. Most of the east end is too far, unless they were to take over that parking lot across from City Hall - that could work as a clean site, other than having to integrate the St. Louis Hotel. Frankly, I'd rather see the art gallery there, could have nice lounge of a re-done St. Louis, as a dining / drinking experience with a visit to the gallery. Doesn't integrate so well with a library

unless they re-purpose it.