Presumed Plan : Soriano will pitch the 9th, Clippard the 8th, Storen in the 7th mostly. Stammen will be part of the mix as well. Beyond that it's a guessing game with Krol having a good shot and Ohlendorf in long relief if he doesn't secure a starters spot. It's likely that another, more experienced lefty will be brought in to round out the relief corps to give Matt Williams some flexibility.
Reasoning on Presumed Plan :
Soriano gets the ninth more by default. He's not bad (3.11 ERA, 1.230 WHIP), he gets the job done (6 blown saves - perfectly average for closers). Plus he's making a ton of money to close. Clippard was one of the best relief pitchers in the game last year, so he gets the presumably next most important role. Storen struggled early in the season but sported a 1.40 ERA & 1.03 WHIP after coming back. Ohlendorf was a pleasant surprise, sporting a 2.86 ERA in relief before moving into a starters role. Stammen was quietly effective.
These guys are all righties so at least one other lefty is needed. Krol, a decent prospect who pitched ok last year, would seemingly get the nod over Abad (pitched worse, older) and Xavier Cedeno (limited look last year and bad history). However one lefty is not enough, especially given that Krol struggled at times and may not be reliable. An important part of the 2012 pen was Sean Burnett. Since the pen did worse without that type of player, it seems logical enough to bring in another. They like JP Howell but he's in a good spot with the Dodgers and is likely to resign. Maybe Javier Lopez or taking a chance with Eric O'Flaherty?
As for the other names out there, Mattheus might have pitched himself out of a spot. Roark and Jordan, who got some relief play, would only figure in if Ohlendorf won the starter's role and left the long relief spot open. Otherwise back to AAA to get starter's innings. Erik Davis likely didn't impress enough to force himself in.
Problems with Presumed Plan :
The main issue is one that isn't going to be overcome. It's using your best pitchers in forced roles in the 8th and 9th innings, meaning they are going to miss out on a bunch of important spots in the 6th & 7th for important spots in later innings that never come. See Me.
Soriano, already a malcontent, is starting to slide in ability as he gets into his mid 30s. Storen is kind of a headcase. Ohlendorf's pitching was good last year but he was also lucky and pitched way better than he has in the recent past so it's tough to see that happening again. Clippard... well I can't find anything really wrong with him, though he could easily sport an ERA a whole run higher. Krol is at an unreliable age and no one else has stood out in terms of looking like they might be a lights out reliever that the Cardinals made it seem like every team needs.
The free agent pool is pretty deep but you will be paying a lot for a reliever, which given the amount of innings thrown, doesn't make pure sabrmetric sense.
My take :
While eventually it'll be done, it's a lot to ask to expect the Nats to be the ones to break from the accepted way of using bullpens cemented over the last 30 years. What's most important is that the pen is filled with decent arms that don't hamper the managers ability to do what he wants to. The Nats presumed plan covers this.
I don't like Soriano, but for one or two more years he might be ok. And don't worry, if he struggles he will lose that closer role, because the Nats can avoid a guaranteed option if he doesn't finish 62 games this year (he finished 58 in 2013). I like Storen and I think the Nats do too. He could be trade bait I guess, but the relief corps isn't that deep. Clippard may in fact pitch 3.50 but with a ton of innings that will still be valuable. Stammen is kind of an undercover hero.
After those 4 it's less clear to me. I think Ross is due for a collapse, but he pitched well enough last year to earn a chance and assuming they sign a SP, they now have long relief depth that I feel ok about with Jordan and Roark. Krol isn't great but he's 22, and as the last arm in the pen he makes me much happier than H-Rod did. The key will be getting that 2nd lefty. A bold manager could work around having one lefty. Clippard has historically been very good vs lefties so you could use a LH early and still have Clip for lefty heavy lineup portions in the late innings. Unfortunately managers rarely see past "Lefty hitter coming up! I need a lefty pitcher!" so that 2nd lefty is needed. Off the top of my head I do like Boone Logan, but that's probably biased by the amount I've seen him pitch.
Sure it'll be costly to sign one of these guys but who cares about cost effective winning. Wouldn't you rather have 93 wins for 150 million than 90 for 100? It's nice to prove you are smart but in the end winning is all that matters, no matter how it's achieved.
Outside the Box Suggestion :
No lefties if Krol doesn't make the team. Good pitchers should be expected to get everyone out. I don't know why it would be thought otherwise. Think about it this way, you want guys who can get lefties out. There are righties that can do that at the fraction of the cost of what it's going to take you to sign a lefty relief specialist. Sign Tim Stauffer (career .251 / .315 / 402 v lefties, .190 / .248 / .281 vs LHB last season as a full time reliever) and take a chance on Jesse Crain (just a good pitcher when healthy) and tell Matt Williams that you have 6-7 good arms in there, if you can't figure out how to get a couple lefties out then someone else should be managing.