I think with either of Davis or MKG we could be a ridiculously difficult defensive team for the foreseeable future. Toughness, depth, length, smarts, energy -- shotblocking, theft from on-the-ball pressure more than gambling, and potentially the bar none best defensive EFG% in the league.
Either player adds a dynamic to our defense that we're currently lacking, but each enhances what we already do well. Davis' effect would be obvious, we need a long armed option that bigs with range can't simply shoot over. Nene and Seraphin give us tough immoveable frontline bruisers with good positioning. Both defend the pick and roll well, challenging outside and dropping back to the interior. But it would be useful to have a mistake erasing defender on the interior who could swat the ball away, and ideally keep it in play, send it to the other end of the court.
This is what Davis does well. Seraphin is a competent shotblocker now that his weight is better and he doesn't foul on every attempt, but Davis is a superlative missile shield type. More than anything though successful defensive teams need a counter to the sort of long-distance bigs who have found a role in the no-hand-check era. Players like Dirk make players like JET/Barea suddenly useful as interior-attacking midgets. Pau, Dirk, KG, Sheed, Timmy, etc. The Stretch Four is a reality on championship teams. If you have no way to defend them you'll have limited success at any top end contests.
As for MKG, the next most important player is a do-it-all blanket defender who can ride herd on the opponents best perimeter attacker, take personal responsibility to choke him off. For this you need length, strength, anticipation, effort, and a frenzy of constant energy to cover your man and your man's man. Here you want to be always in the way to present interior penetration, with long active hands that stifle interior passing. Make them swing it sideways, not through you.
This is Kidd-G's metier. In fact he was so active a defender in the NCAA that at the NBA level I'm concerned he may suck fouls for a while 'til he figures it out. This has happened to our own Chris Singleton who has become frustrated and passive with rookie hazing by the refs. He hasn't yet earned the reputation of an NBA caliber defender, thus he won;t get the benefit of the doubt, thus he's gotten somewhat sour with the refs, which won't earn him any calls.
The difference being that KiddG is much more naturally gifted with lateral speed and anticipation on positioning. Yes he's handsy, yes he's too physical on the perimeter (where you can get away with it on the interior). Granted, the games where he disappeared this season often came when he picked up quick fouls, he doesn't know how to play at half speed, it's all or nothing. But adding him to a squad that earns a reputation as a tough defending squad, and being that he'll have name recognition as a Nat'l Champion, and will be picked early in the draft on this reputation primarily, I suspect he may garner some benefit of the doubt cushion.
Either way, when he's active, and with his clear competitive joy I suspect he won't sulk for long no matter the foul situation. I can see teams having a devil of a time trying to run what they do. He can pressure the ball, pressure the shooter, trap the pick, collapse to slap away an unprotected ball in a crowd, and will earn hustle points from referees who see him diving after every loose or contested ball. And no fast break could count on an easy basket at the other end as like John he rarely gives up on a play.
Strange to think that defense could be the hallmark of a Wizards team, but the way things are shaping up now it looks like we could be downright nasty int his regard. With Johnny Ballgame taking a leadership role on this front as well.