There are a wide range of rivaling task management methodologies. The all use both calendars and lists, but they can vary quite a lot when it comes to the principles by which you bring things forward and get things done.

Doit is a VERY capable app. It can already handle many more methodologies than just GTD.

When I saw the blog post about the Daily Planner, I immediately thought of DIT - Doit is aiming for doing DIT (Do It Tomorrow, Mark Forster):

http://markforster.squarespace.com/do-it-tomorrow/ (the author's own website)
http://markforster.squarespace.com/forum/post/551255 (a very clear comparison with GTD)
http://www.thesimpledollar.com/review-do-it-tomorrow/ (a review)

The reason I am writing this here is the fact that Doit is so generally capable, and can handle so many methodologies, that it is almost a pity to describe it (in your own marketing) as an app just specifically for GTD. It could be described as an app that manages GTD (David Allen), DIT (Mark Forster) and standard project management inspired personal task management - and any mix or combination of these.

Of these three, DIT is the "middle" option. It tells you to plan exact tasks for TODAY ONLY and follow that list exactly. It is "holy". All the rest goes on the asap list (called Task Diary, but is essentially a Next list).

As you know, one of the extremes is project management inspired methodologies, where you create schedules for days or even weeks or months in advance for exactly what you are going to do when. The other extreme is GTD, where you simply do not schedule any of your solo work at all. You select tasks entirely as you go, entirely depending on the situation and your "intuition" (as David Allen calls it, i.e. considering context, energy, priority etc).

Doit can do the whole range of methodologies already as it is, and you could easily call yourself an app for all of these methodologies even today.

If later you wish to develop stronger features for the DIFFERENT aspects of these methodologies, it would probably be things like:

- For standard project management inspired scheduling: Different calendars (colors) for different projects, goals etc. Perhaps critical path etc..

- For DIT: a strictly last-thing-last sorting order for the Next list, where unfinished tasks from the Today list are looped back to the very last position on the Next list when unstarred (regardless of context, priority or anything)

- For GTD: More convenient/powerful quick filtering for situational selection of tasks (as discussed in many other posts)

Interestingly, Mark Forster has also created another, much, much simpler methodology than DIT (and than GTD). This is called the Final Version. It is based on always picking exactly THREE tasks from the "next list" in a very particular way. In other words, in the Final Version, he does not recommend planning tasks for the whole day, as in DIT, nor does he recommend picking less than three, as in GTD, where you often pick even just one or two at a time from the Next list. The whole idea of the Final Version is to always pick exactly three: the first task on the list, "the first in line", no matter how horrible or scary it seems, and then pick one more from anywhere further down that you want to do before, and then one more even further down that you want to do first of all (i.e. you execute them in the reverse order from how you picked them). This, he argues, ensures that nothing is left undone forever on your list, and that you still get all the important things done. http://markforster.squarespace.com/

Personally I like Mark Forster a lot, and have read his books, but I am quite happy with GTD and do not really actually use very much of DIT or the Final version, but it is good stuff, definitely worth checking out and considering.