The Master Motors lot at Washington & Grand (future Washington Point Park and/or restaurant with outside dining is a problematic shape - wondering if that made it less attractive to potential developers. Also, maybe pressure from the cops who park all over the Grand St sidewalk and would have lost their easy parking? Remember, this is the Shameless Speculation thread...
Natty Garden is a great example of adaptive reuse, I hope he does well there.
666 Washington Ave looks like its zoned R6A with a C2-4 commercial overlay - same as the rest of the Washington biz district. Which means (generally) it should be eligible to get a multistory mixed-use building with residential over stores/restaurants/etc.
If Propertyshark's assessment that it's 34K sq. ft buildable as zoned - in my estimation that's a 5 - 6 story building with a total of approx 30 apartments. That aligns with the new Hello Living building going up at 518 Washington just two blocks down - an FAR of about .35 or .36. That building is 6 floors + penthouse with parking (and hopefully storefronts) on 1; five floors x 2 each ~ 1,000 sq ft apartments, plus a 650 sq ft penthouse - on a 3,800 sq ft lot.
Maybe the lot at 518 was more attractive because it's got three street sides, a bit closer to the C station at Fulton and Washington, and no obvious existing business on it. But the much larger 666 - 674 parcel (five standard lots!) has got to be tempting to both seller and developer. 30K sq ft of apartments at ~$600/sq ft is $18MM gross revenues...unless there's 60 years of leaky gas tanks contaminating the ground beneath...