For me, the biggest feature I'd like to see is collapsible nested lists. I use a checklist app to pack for travel, and it's really handy to have categories such as "international" that I can collapse when they're not relevant.

It's also useful to have an option where checked items disappear from the list, so you only see the stuff you still need.

Oh, and of course it would be nice to have multiple lists that you can switch between. That should be pretty easy, just open a different file (after saving the checked/unchecked status of the current one).