Suggestions for extending the planning capabilities of the app get brought up from time to time. Sometimes it comes in a form of hierarchical projects, sometimes it’s dependencies between tasks. Here are some of my thought on this. I’m sure there’s more on the forum, these are just the top search results.
In short, @manu is spot on: the complexity vs the usefulness in the context of a GTD app of complex planning makes similar features unlikely.
Regarding the specific mechanic suggested by @Hydro, it does look simple when creating the items or assigning them to projects, but a much more difficult problem would be to visualize the resulting project and allow for editing of the dependencies in a reasonable way. This is where the feature quickly gets to much out of the scope of improving core GTD capabilities.