Hello, thanks for Checkvist, its keyboard driven, modal approach it absolutely delightful, and it has no rival in the productivity/outliner web space.
Did you ever consider implementing clones/mirrors? Basically a list item can be in more that one place at once, and when editing one of them the changes will be visible in all instances. It could use a similar mechanic to linking, using something like (( to mirror an item.
In the outliner space, Workflowy and Legend (formerly Moo.do) have it.