I suggest using google docs or other cloud-based software for coordinating tasks and work with everyone else. Simple e-mail won't work.
Also, try to set a clear deadline for each and every task. Draw up a list of clearly defined tasks (this is really important, don't have people waste their time and motivation on figuring out what they're actually supposed to do) and have everyone assume responsibility over a part of them - and make them do it themselves, assign tasks yourself only when it's necessary, ie. when there's some tedium that nobody wants to do. People have it easier to motivate themselves when they do something that they chose to do.
fe.
Badly explained task: "Dialogue writing: browse the list of NPCs, pick one and write some dialogue"
A clearly defined task: "This week we're supposed to be done with dialogue for Nix, Robert and Liam. These are fairly important NPCs so you'll need to put some effort into it.
Next week we'll doing a bunch of minor NPCs but that'll be a breeze, just a couple of lines for each. I'll provide extra details when the time comes but you can already start thinking about some lines for shopkeepers, gardeners, soldiers etc. if you can spare the time"
Next, try to draw a hypothetical timeline and try to divide the tasks accordingly and in a logical fashion so that each week has a similar workload to the next one while going along the plan without setbacks means that every week an important part of the job gets done. In your case I'd believe that you'd want to have fe. a quarter of Tarant done in one week, another quarter in next week etc. Once Tarant is done you focus on a village, then on wilderness areas or something. You get the logic.
It serves many purposes, one of them being that if people work on different areas of the project they'll pester each other or you for resources or advice that will be considerably more difficult to provide. If all of you are working on the same quarter of Tarant you'll face less problems.
Be an arse in general. If someone's falling back, pester him with e-mails. If he keeps giving you the "sorry i've got a cold/rash/uni/life" excuse, drop him from the team and find someone else ASAP. That's where the cloud-based software really shines - you already have a complete database of what's done and what's still pending completion. That database required no additional effort as it was a part of everyone's job and will make miracles when you need to fill someone in the project's status.
Also, don't be afraid to kick someone out if he's not exactly falling back but is generally underperforming, ie. the job he does is of sub-par quality or if he's a pain to communicate with. Such people are a hindrance to work with and usually mean that everyone else must do at least part of the stuff for them (fe. fixing grammar in dialogue, bugfixing scripts, remaking some graphics etc)
You really need to focus much of your effort on proper management. Whenever there's a project to be done good ideas are ultimately secondary to good management - after all, it's better to have a finished mediocrity than design a masterpiece and abandon it somewhere along the way.