Part of the 3PT defense is poor defensive communication. Guys get lost too often and don't know where they are supposed to be, or they are practicing poor defensive fundamentals such as sagging off shooters and being sucked into the lane when they shouldn't be. Sometimes the effort is there, but misplaced defensive effort on defense just equals frustration.
In terms of 3PT shooters, instead of guys like Korver (who is getting older and not much of a defender) or Reddick (who will make a decent amount and can't adequately defend SG's because of his lack of length). A guy like Martell Webster would be a good player to go after as he can do similar things to them. Washington probably wants to keep him now though with how he's played for them and opened up the floor for Wall. A guy that can shoot coming off screens and therefore attract attention would help, but isn't as critical as three point defense and the ability to stretch the floor with a big.
Stretch big has been needed for a while, but the problem is that with Blake being a PF, the need is a stretch big that can at least adequately play C, and that is much harder to find than a stretch PF. A stretch PF is okay, but you don't want to be playing two small PF's (Blake + the other guy) on defense. The options are not as plentiful in the stretch C role. It is also true that a big man that can score would be helpful so that when the bench is in, it isn't iso Crawford time unless they get out in the open court. A guy like DeJuan Blair would be nice, but again, he will need to play more than just those minutes and you'll have to put him on with Blake.