Idea: GTD version of Lightning

Been using Thunderbird with Lightning, and its a bit tough as I'm trying to use it GTD style. So this idea is to tweak lightning so its Task features become more GTD friendly.

• First off, tasks should have sub tasks
• Also, they must be allowed true categories, ie one task can be in multiple categories. Right now only one category is allowed.
• Search Tasks feature should be there
• Group by multiple attributes would be useful. Eg group by category (Next Action, Someday/Maybe) and then by context (@Home, @PC, etc - which I use the location field for)
• Link tasks to the messages or events that they have been converted into.

.. and some more that i cannot remember now.