there are a lot of those smaller half- or quarter-block parking lots downtown that are ripe for development. i've often thought that space next to the Idanha would be ideal for a 20-25 floor skinny residential tower; the rendering in that pdf shows how nicely it would fit.
another great location for a new tallest is on 9th across from Hotel 43. BoiseAirport had a rendering of how a high rise could fit that space nicely.
i also think the Hotel 43 lot with the outdated garage is a candidate for redevelopment.
the Idaho Press has an article today about some of the proposals, though it's got a lot of negative NIMBY tone to it. apparently NIMBYs would rather see 50 homes gobble up more farmland and add more cars to the road instead of a nice dense high rise next to the greenbelt.
the article also has this rendering of the Ovation I hadn't seen before: