Rather than simply linking to a list/sublist, which embeds a hyperlink in my existing list, a real killer feature would be to actually embed that sublist in my current list. This would be have a lot like a symlink, with the key feature being that if I edit something in the sublist that edit is reflected everywhere in Checkvist that that sublist appears.

Example use case: If I am listing out travel ideas by Geography as follows:
- Italy
--- Wine tasting
- Puerto Rico:
--- Caving
-------- Tour operators information
-------- links
-------- other information specific to puerto rico caving, etc
--- Scuba Diving
--- Beaches
... etc

And I also have another bucket list, which is organized around Activity Type as follows:
- Natural Wonders
- Rafting trips
- Caving trips
- Diving trips
- Sailing trips

What I would really love to be able to do, is embed the Puerto Rico Caving sublist (in the 1st list above - Geographies) into the "Caving" section of the second list (Activity Types) above. This allows me to enter more information and flesh that sublist out further, regardless of whether I'm currently thinking along Geography or Activity Type. Changes are reflected in both.

At a meta-level, this allows lists to go beyond 1 "hinge" of data. I can effectively organize my information across 2 criteria/dimensions, rather than being limited to 1 (I can organize specific information about caving in Puerto Rico so that it appears on my list of travel ideas organized by Geography as well as my list of travel ideas organized by Activity Type). More generally, this allows massive flexibility.

It will also form the backbone to some pretty amazing sharing features. For example, I could share a sublist with someone and instead of them having to view it as a "list", they could actually move it around within their own list structures. Maybe I share a specific Todo list with someone (i.e. fix site CSS) which has multiple sub-tasks. Instead of having to view this as a full list, they can actually take this and move it around their todo list (however they may organize it). Since all it really is, is a symlink to my subtask-list, any edits they or I make will sync.

Workflowy implemented something similar to this, focused only on sharing. They allow you to share a sublist with someone, and for them to move that sublist around in their own hierarchy with ease. What they have not implemented and what could be extremely interesting, is sharing that same sublist with yourself. This would allow you to move it around your own lists and hierarchy, so that even if a list item should appear in multiple places you don't have to duplicate it.

Implementation-wise, the only real hairy piece is circular references. I Imagine this is a nightmare for workflowy since their users just have one big mega-list. However, for Checkvist, it could actually be very easy: don't allow people to embed/symlink any lists of sublists within one List. Only allow symlinking/embedding sublists across separate Lists (full list objects).