I use FT mainly for writing — letter, reports, proposals, anything that's more than a short note (I use NValt for reference notes). I also use FT as a tool for planning things that are clearly linear, or for things I have a fair idea how to tackle already and just need to order my thoughts. For more complex planning especially for things that don't have an obvious order to them, I tend to use iMindMap initially, and then once I have a sequence of events, I move to FT or OmniFocus to order the steps and record their execution.
The main advantages of FT for me are: (1) the clean uncluttered interface; and (2) the ability to hide (fold away) bits of the document that are not relevant at that time. The word-count feature is also useful. Essentially FT is my thinking space. If my writing is destined for others to see, normally the text then gets transferred to Pages or InDesign for formatting.
I would use FT as my task manager but for one feature it lacks and that is the ability to warn me of an upcoming deadline. I've thought to write a script, or some UNIX tools, to parse an FT document and email me a daily report, but I can't justify putting in the time to get it working whilst OmniFocus does a good enough job (even if I don't use half the features of OmniFocus, crave something much simpler, and would prefer a text-based system to guard against redundancy).
Other than some way of creating an alert for coming due-tasks, the feature I'd most like to see would be some way of viewing images in amongst the text. I mainly write about images, so actually having them viewable in the document would be very convenient.
If you were to consider supporting images, the way I would suggest is simply to use some sort of hyperlink consisting of a section header with and appropriate suffix (e.g. ".image") immediately followed by the path to the image file, something like:
The image itself would therefore not be stored in the FT file, which would remain purely plain-text. But the FT app would display the image, at say some default width and height and centrally justified, immediately below the path (rather like the answer appears in a ".calc" section). Images sections would be foldable. Something pretty basic.
Hope my meanderings are of some help.