"What would you trade for __that's actually available? They have five starters and a couple extra in AAA. They have a deep bullpen. Bernadina is playing/hitting better and takes care of CF as well as anybody you'd get. If Werth, Z'man, Desmond start hitting at their career levels, where's the Big Obvious Hole to fill?"
There's no holes! This team is perfect!
Ignoring the bench for now (a team shooting for maybe a WC doesn't need to trade anything decent to fill a bench spot) let's agree that there are some holes to fill. Where? Not C, 2B, 3B or RF (even though it is a hole right now - it's not gonna be unWerthed anytime soon), but the other positions? Let's take it from biggest hole to smallest.
(and let me note I personally don't think the Nats should bother addressing any of these issues this season. Let these guys play and see what happens. Maybe you get lucky. A couple of these spots have minor leaguers ready now or soon. No reason to go out and get someone. But I think the Nats aren't a playoff team and shouldn't worry about losing a couple more games. Your mileage may vary)
Shortstop - Boswell says if Desmond started hitting at his career level SS wouldn't be a hole. Here is Desmond's career line (from all of <2 years of major league hitting so you really shouldn't expect him to get back to it) : .257 / .296 / .385. This is why Boswell is not a GM. A guy that never gets on base that maybe hits 10 homers in a year? That my friend is a hole. And that's factoring in Desmond's crazy hot 20 game stint from a 2009 that seems so long ago. Ok, ok you say. He's a SS he doesn't have to hit well as long as he's fielding ok. Fine. But it's not that he's hitting below average. He's the worst hitting SS in the MAJORS. By a good amount too. BIG OL' OBVIOUS hole.
Centerfield - "Bernadina is playing/hitting better and takes care of CF as well as anybody you'd get." True if you add in the all-important phrase "right now". On the offensive side Bernadina had a hot couple weeks that's keeping him at average offensively, but I don't see a reason he can keep doing even that well. But he's rather similar to Desmond in that his minor league numbers suggest more a guy that had a fluke year, than a good player waiting to happen. I don't think he's as overmatched as Ian, but I think last year's numbers (.246 / .307 / .384) are more in line with what you should expect. And despite the flashy plays, he's still not playing great, maybe not even good centerfield. He's a 4th OF. A player you can plug in for a week or two and hold the fort with. A pinch runner. He's not a starter.
The Nats may not be able to get a better offensive player than Bernadina, but defensively there are some above average guys out there. It's a hole. Obvious if you take a second to look at it. Big? Maybe not Big. Medium-sized.
Left Field - In the past month Laynce Nix has hit .209 / .250 / .388. His major league production bounces around but I think the "Low-average, decent pop" handle works. Historically (though not this year) a decent fielder, you could win with Laynce as the 3rd of your 3 outfielders. But he's not. Right now he's carrying the offensive load for the OF. It's going to be hard to find a CF that's offensively great, so LF is going to need more production, especially as Werth gets older and ... less productive? Geez. Anyway. It's a hole. A small one. Not very obvious because a hot May makes his stats look very good. But it's a hole.
First Base - For the first 30 games Mike Morse was terrible, low average, no pop (3 XBH), just a waste of a spot in the lineup. For the last 40 games Mike Morse has been one of the best hitters in all of baseball. 14 homers. An average over .340. He's been slipping a little bit in the average dept lately but the power is still there.
Which is the real Mike? Probably neither, but given his performance last year it seems closer to "40-game" Mike than "30-game" Mike. This isn't really a hole, but after less than half a season I'm not ready to consider this resolved just yet. A non-obvious piece of ground that needs a bit more investigation.
The Nats are in much better shape now than a year ago. They have a great pen. They have 5 starters that can hold their own (or better in the case of ZNN). They are much better on D. They have an offensive team that's not an embarrassment. Most of the holes they do have have potential solutions in the minors (Lombardozzi, Bryce) But no holes? Come on.
(also I know it KILLS you out there that Lannan put 14 guys on 5 and 2/3, only struck out 2 guys and only gave up 3 runs. Too bad! It's what he does. Go Lannan, go! Scatter those singles!)