If you're serious about supporting GTD methodology, here's an idea: Have a feature to process the inbox. Just click "process inbox", and the user is walked through the process as outlined in the GTD book. The inbox item is displayed, and the user is prompted: Is it actionable? Yes/No. ... If no, the user is asked if it is (a) trash, (b) might be need to be done later (Someday), or (c) useful information that might be needed for something later (Reference). If the user answered yes, they are then prompted: "Can it be completed in one step?" If no, they are asked "What is the desired outcome?" The answer is turned into a new project (the text of the inbox item could be displayed as a default that can be modified, to save time), and the user is then immediately prompted with the question, "What is the next action necessary to make this happen?" The answer then creates a next action for the project. Etc. etc. etc.

The GTD processing method is a flow chart. Most straightforward thing to implement in a computer program. I'm not a programmer but I could probably do it in Python if given enough time. Given the popularity of GTD, I'm shocked that there are no popular apps to my knowledge that do this.