I once used a GTD app that supported something like that.
In the app you can select multiple actions and then set them to parallel. This way all actions of this group show up as next action, but there also can be more actions that are sequential to the action group.
It added a lot of complexity that the programmer had to deal with. I didn’t use it that much, because my projects seldom are planend out in such detail that I would / could make use of this feature.
My personal opinion is: Nice to have, but probably overrated
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.