I am not familiar with GTD, but I would find an option for a calendar view to be very useful. Besides have tabs for Focus and Context, it would be nice to have an item for a calendar to see the big picture of what you are doing: for the day, week, month, and year with color-coded lettering/blocks for Contexts, similar to Google calendar. Right now I am having to use both Google and Doit.im, but it would be nice to just use Doit.im, at least to be able to see it right in Doit.im.
Also another feature that would be tremendously useful would be a notepad option. Although the Inbox and Someday options are useful, sometimes it takes too long to fiddle with that when you just want to write yourself a note to input correctly later. This is useful when you are in a meeting, or talking face to face with someone and just want to jot their name down in order to schedule them in later so as not to be rude while you figure out your app. Thanks for your consideration!