I run a business integrating security trading software into some of the nation's largest brokerage firms those plans, using the previous posters words are "massive projects" and I would not expect TD in its current iteration to handle those projects.
I use TD to manage my family of five. And even with that limited scope I find not having sub subtasks very handicapping. Without them TD quickly becomes a to do list rather than a tool to help us become more effective and efficient in the things we need to do.
TD, you should reserve unlimited subtasks for future upgrades, but adding one or two additional levels to a paid Gold subscriber does not seem like an unjustified request.