The Thomas Building is a great candidate for residential conversion, but it's forgotten due to its location (and with the surrounding buildings/narrow streets it doesn't get a lot of sunlight). I like your idea, but a plaza probably isn't needed in this area; one block north is AT&T Plaza, and one block south is Pioneer Plaza. Building a 5-6 story terraced residential/retail structure on this block (connected to the Thomas Building) with a prominent architectural feature on the corner would probably be the best use (including underground parking). There's not much retail in this area. There's already a small plaza on the west side of the Thomas Building (and behind) which could be enhanced for residential or retail uses.