With the recent teasers for the Death Guard things seem to be finally looking up for the minions of the Plague God. The Chaos Index version of the Death Guard's rules are deeply frustrating for those of us who happen to be existing DG players, with the Legion losing access to all manner of units, like Bikers, Obliterators and Terminators. It's not that the new stuff is exactly bad, but when you have a unit of ten Nurgle Havoks specifically made because the Traitor Legions book made them Relentless, suddenly finding you're not allowed to use them at all is not good news.
The approach I've taken with my own warband, the Dereliction of Duty, is to run them as Alpha Legion. I imagine it as being a warband that releases a tailored genetic plague into planetary populaces that turns the citizens into brainwashed zombies, and over time the use of such methods has brought the Warband to Nurgle's attention. From a mechanics point of view, this works really well. The Alpha Legion 'Hidden in Plain Sight' trait gives the enemy -1 To Hit at over 12" range which in this case we can think of as being due to the classic cloud of flies. It's a strong trait, and one which has the effect of making the Warband more durable under fire, which is the sort of thing you want as Nurgle. The 'Forward Operatives' stratagem can be thought of as piles of rotting corpses turning out to not be as dead as the enemy thought.
The Alpha Legion trait of course also stacks very well with Miasma of Pestilence, giving enemies a total of -2 to hit at over 12" or -1 if they're closer. This is really evil because of how re-rolls work- since they're applied before modifiers, a BS 3+ unit with a re-roll to hit can't re-roll a 3 (because it's a hit at that point) but then has it reduced to a 1. May the Dark Gods have mercy on any fool to Overcharge under such circumstances. This is really nasty on a unit of Obliterators, especially since you can teleport them in with a Terminator Sorcerer for a perfect firing position.
We should also take a quick look at Grandfather's Blessings, the Nurgle stratagem. Returning D3 wounds or a single slain model to a Nurgle unit for 2CP isn't exactly amazing, but if that model is an Obliterator in a unit that's already proving very hard to shift it can be a really nasty trick, especially if you have CP to burn because you brought a Battalion of Cultists (and if not, why not?)
For the Warlord, you could certainly use 'I Am Alpharius' and have your actual Warlord as a small Daemonic tick that flies from one host to another Tamurkhan-style, but my personal pick is 'Unholy Fortitude' because that's about as Nurgle as it gets.
The other very solid Legion Trait for Nurgle warbands is Night Lords. 'Terror Tactics' knocks the LD of units within 6" down by 1 per unit, to a maximum of -3. As far as I understand it, the -3 'cap' is only for reductions from this trait, so you can increase it further with the -1 from an Icon of Despair and the -1 from Raptors. Neither of these two stack, making the theoretical maximum penalty -5. You could also, of course, go with the 'Lord of Terror' Warlord trait to make any Morale tests even more punishing.
This is one of those approaches that is either going to make your opponent weep with terror or laugh contemptuously. Against a Guard army (if you can assassinate the Commissars), Orks or similar, it's potentially extremely effective, whist MSU armies with high LD will barely notice a lot of the time- though if all the cards fall right, even LD 9 units become LD 4 and potentially risk Morale failures from a single casualty. It's particularly nice against models like Noise Marines or Loyalists near an Ancient who trigger attacks when they die, or Apothecaries who can heal slain models, since Morale losses don't count for either of these abilities.
Using this army effectively really hinges on getting rid of any Morale-boosting enemy leaders quickly. A flying Daemon Prince with Smite and Infernal Gaze is a handy tool for this- often said characters will be standing between several units so if you can get a Smite+ charge combo on them then you'll probably be well-placed to spread Nurgle's gifts. If the character is actually in the middle of a unit then it's a case of psychic sniping with Gaze or Gift of Chaos. Of course you could even try taking Renegade and Heretic allies to get Marauder snipers, but given the attendant costs and their unreliability it's not a gambit I'd recommend. Deep-Striking Terminators or Obliterators could also be employed to try to clear a path to the target character through brute force and firepower, or even just gank one standing right at the back of a formation, though most players only make that mistake once.
Since you're going to be much closer than the Alpha Legion approach, it's worth remembering the 'In Midnight Clad' stratagem, which is another one which stacks with Miasma, but more importantly can be popped when a unit is targeted, greatly discouraging Overcharging. Since basic CSM squads really don't work well as Night Lords because they need to get close to make the Trait work, that's another reason to take the Cultists to rack up some cheap CP. Did I mention I really like taking Cultist Battalions?
I'm hopeful that most of this stuff will turn out to be a short-term measure, acute rather than chronic, if you will. The Death Guard Codex will surely have more units in it, some form of Terminators, maybe a Havok analogue, hopefully some sort of Plague Bikers which will allow us older Nurgle players to come back to the fold. But for now, at least, with a little imagination the forces of the Plaguelord are in rude health... er.. viable.