Reasoning on Presumed Plan : Despite being 35, Werth put up another great line offensively. He hits for average (.292), maintains decent power (16 homers, 37 doubles), and most importantly gets on base at an elite level (83 walks and a .394 OBP) which is a rarity in the Nats line-up. While no longer a fast runner, he's still a smart one on the basepaths. His defensive range is shrinking (he hasn't been a good OF since coming to the Nats), but 1B is unavailable so sticking him in the corner that he's most accustomed to makes sense.
Despite being only 21, Bryce put up another decent offensive season, and expectations are still that when he finally has a year where he's healthy he will break out. Primarily the Nats expect that he'll mature into a 30+ HR hitter, though hitting around .300 or learning patience to get on base with an OBP near .400 remain hopes. Either way, even if he simply repeats the "disappointing" 2014 numbers over a full year, Bryce is an above average bat worth starting. Plus with LaRoche gone he's your only power lefty bat. On the basepaths Bryce was noticeably... less enthusiastic. That could be coaching, but is probably due more to his hip injury. Still he's far from an oaf. Defensively he has yet to fulfill the promise of his teen years when it was thought that his athleticism would carry him if his he didn't take to the position. He's passable in left, albeit with a cannon for an arm, nothing more.
Given that Werth and Bryce are primarily offensive players and CF is primarily a defensive position, it makes sense that a defense first guy would play that spot. Despite a bit of disagreement on how good Span is, most observers think he remains an above average fielder. That alone would probably get him a starting role but Span busted out last year with the best year of his career hitting over .300. While it's unlikely to happen again there's no real sense Span is on a decline, meaning he's likely to provide no worse than average offense and likely slightly better.
McLouth remains in the bench mix by default. He's a lefty bat when the Nats don't have many and more importantly he's getting paid 5 million next year. Steven Souza dominated AAA pitching in ~100 games last year and at 26 in April, has reached an age where you either use it in the majors or lose it. Taylor is likely the future CF but given that he played only ~20 games in AAA last year and was unimpressive and that he's two years younger than Souza, he gets the everyday job in Syracuse.
Problems with Presumed Plan : There are some issues here. I see the biggest one being Werth's fielding. His range has diminished to the point where he's got one of the smallest ranges of a regular RF in the majors. It's not going to get better either as this isn't an injury thing, it's an age thing. Span can help cover that issue but not as well as he could if he didn't have to help Bryce out too. The general consensus is that your worst OF plays LF meaning Bryce and Werth should flip.
Even in optimal position though the Nats probably won't increase from their position of the 23rd "rangiest" group which isn't good. They get a slight bump for having good arms in the corners and not making a lot of errors but it's still a below average group.
Offensively the biggest concern is health. Werth did manage to play a full season, but missed a big chunk of 2013 and half of 2012. At his age you have to expect 120 games, not 150. Meanwhile Bryce has played 118 and 100 games the past 2 years. If both of these guys miss significant time again that may dramatically hurt the offense.
There are also individual concerns at the plate. Werth's power is dropping. What was once blamed on an injury is clearly now age related and while he can still bang out a double, the homers are getting fewer and farther between. His HR/FB rate goes 21.1% (2008), 19.3% (09), 14.3% (10), 12.3% (11), 9.4% (14) (Both 2012's 5.3% and 2013's 18.0% seem fluky and out of pattern). We may be able to blame Bryce's decrease in power on his injuries, but Bryce's K rate spiked dramatically in 2014 up to 26.3% where it had been usually below 20% in most of his stops. If he can't adjust to whatever is going on now his ceiling drops. Lastly Span basically got a 20 point bump in everything due to a 20 point bump in BABIP. Expecting everything to fall back is the smart play.
On the bench you have no reliable hitters. Souza is a big talent but a big question mark. We've seen AAA killers be unable to produce in the majors before. McLouth was a big nothing last year and it's hard to see him as anything viable as a long term replacement if there are injuries. Michael Taylor, as noted, hasn't hit above AA and didn't look like a major league hitter in his cup of coffee.
My take : There's no good reason to deviate from what was done in 2014. Even if you could with Bryce and Werth, they are good players. You play good players. The Nats could move on from Span, but Souza doesn't fit the "great defender covering for corner OF" description and Taylor isn't ready. At a reasonable cost, it'd be silly not to re-up him.
The biggest issue is the defense and yes the defense is suspect. But it isn't terrible and given that offensively the group should be Top 5 if healthy, you live with it. Should they flip Werth and Bryce? That is probably optimal but the whole RF over LF thing is mostly an arm issue and Werth's arm is still good. Flipping would hardly make a difference. Beyond 2015 it's only going to get more dicey as Werth calcifies completely and there is no place for him to go, but for next year, I'm not overly concerned.
Offensively I don't see any issues. All those points above are valid, but unless something crazy happens it's still a good offensive outfield, with potential to be very good. I expect Werth to hit about the same, maybe a homer or two less. I expect Span to drop back to his usual self. I expect Bryce to hit a bit better, primarily with a bit more power. That would probably balance out. But if Werth has a minor bounce back and Span stays roughly the same? Or Bryce does take off? That OF could carry a team.
I think the bench is about as good as you are going to get - provided that the Nats use Souza as the 4th OF and not McLouth. (and for god's sake do not play Frandsen out there). I'd prefer a Eury Perez type (I know he's gone) as the 5th but McLouth is stuck in that position. Oh well. It's a 5th OF. As for the first in line in case of (likely) injury, I'm not worried about Souza providing some contribution if called upon. His AAA profile wasn't like the Tyler Moores of the world, where they K'd to the point you knew major league pitching would eat them up. Souza is a better hitter than that.I suppose if you want to eat McLouth's salary, going after DeJesus for a 4th/5th would be my play. I don't see the Nats doing that though.
Outside the Box Suggestion : You can't do anything with Werth. That contract, his age, and a lack of alternate position makes him an OF in 2015. So you can do two things - challenge deal Bryce, or upgrade Span. Challenge dealing Bryce is hard because frankly there aren't many players worth what Bryce is even often injured and not hitting up to projections. Certainly no minor leaguers I can think of. So you have to look for older players, with at least a couple years of team control who aren't paid too much. Could you trade him for McCutchen? or Stanton? I suppose...
So that leaves upgrading Span. If you are going to do that a sign and trade of Span makes the most sense. So you sign Span and you deal him. We've talked about this before but the Tigers may be the best fit. However if you could spin him to the Cardinals for a decent prospect arm, which they always seem to have, that makes the most sense. Why? Because you are probably about to lose your best one.
You send Giolito to the Brewers for the Carlos Gomez. Crazy? Sure, but sometimes you have to go all in and there isn't a better outfielder that you are going to get. He is a better defender than Span, a better baserunner than Span, younger than Span, and you know how Span could hit .285 with 30 doubles next season? Gomez will do that too except instead of hitting 5 homeruns he'll hit 25.
If you are wondering why Giolito, by all means start with Taylor + Cole if you like, but I'm not touching that if I'm the Brewers. Gomez is paid a silly team-favoring amount (8 million in 2015, 9 million in 2016) over the next two years. An amount that lets the Nats do some long-term deals while getting better during that time. Still Giolito? You see a likely Top 10 prospect in all of baseball, probably Top 3 pitching one. I see a 20 year old that has already had Tommy John who hasn't progressed into High-A yet. If all goes on schedule he's in the rotation full-time 2017. IF ALL GOES WELL. In my head I think the likely story is he does get there, gets injured again, has another Tommy John and is back full-time in the rotation in 2019. He may be dominant then but hell that's FOREVER away. Let's win now.