For Portland to take the next step, they need players (at any forward/center position) who can shoot the ball from the perimeter as well as play defense. Right now, if they want to get better defenders like Plumlee, Kaman, Davis or (with some projection) Vonleh on the court, they're punting on shooting which kills their spacing. If they play their (few) front court shooters like Aminu or Leonard, their defense is going to suffer more.
They really don't have one killer lineup that can generate both spacing and defense. To be a top team, they need at least one 5-man unit that works really well together and then hopefully piece together some depth, even if it's flawed, behind it. They have a start with a great backcourt and good pick-and-roll players like Plumlee and Davis. They just need at least one more three-and-D guy who can ideally play forward (or center, but that's much rarer).