For todos you could just use an iCalendar file full of VTODOs [1]. It's a shame that the iCalendar standard is poorly and incompletely implemented even in calendar programs and almost always missing from organizer programs that are basically todo lists.
ie basic functionality I personally look for that I don’t think this would support would include:
* Projects with hierarchies
* Reminder dates
* Tags and contexts
* Multi-Step tasks