Feature request: create type of sequential subtask

Some tasks consist of a set of subtasks that may be done only step by step. I don't find any way to mark a subtask that depends from the other previous in the step list. How can I do it, if I don't have any assumptions about subtasks duration?

After short discussion Luca Invernizzi said on 2010-05-05:
To sum up, you're proposing to have two kinds of subtasks: concurrent subtasks (like they're now, they all show up in the workview) and sequential subtasks("Starts after " [list] "will be done.").
That's an interesting idea and some software similar to GTG does that, so you should open a bug about that (one of the biggest problems with GTG is to keep it as straight-forward and easy to use as possible, which means that every proposed feature has to be discussed :) ).