Quick Entry would be highly interesting as a separate app (meaning that I could make a quick entry without opening TaskPaper). I've done this myself, using Applescript and Keyboard Maestro (it simply prepends my entry into the .taskpaper file). But if you could make a tiny menu bar app that is running at all times, is invoked with a keystroke, and allowed me to choose a project to insert the entry in, that would be amazing.
Tab views would be interesting if each tab could be a separate document. For example, my main TaskPaper document is called Focus.taskpaper. My secondary one is Incubate.taskpaper (for less urgent projects and tasks) and my final one is Websites.taskpaper (which is dedicated to website projects and tasks, which can get quite large). It would be great to have all of them opened as tabs in one window, in the same way that Safari handles web pages.
If I may, I'd like to list a few feature requests:
It would be great to have a keyboard shortcut for Toggle Projects Sidebar.
Add a weekly review system. It is one of the few GTD features that I miss from OmniFocus. I have come up with a workaround using Applescript and Keyboard Maestro. If you would like to see how I use it, let me know.
I would love to have a native command (with keyboard shortcut) to remove all tags from the currently selected item(s).
It would be amazing if there was a alarm tag (like the one that Taskmaster has) that would trigger a OS X notification when the task was due. For example: @alarm(2015-11-05, 1:00pm)