I've experimented with several interfaces myself. One, perhaps the most natural to integrate, would be to simply define a "relates to" tag in workflowy in addition to the hashtag (in my own thoughtstream and on Hackathonprojects.tk, I use <-> or, more concisely, ~ followed by the title of the related idea). When you type the relates-to tag, just like the hashtag, a dropdown showing possibly related ideas (or even better, a grid like when you're adding tags to a question you're asking on StackOverflow) would appear. Ultimately, this could end up using a fuzzy conceptual search/intelligent autosuggestion of related ideas before you even start to type. I've spent the past year on designing this latter set of algorithms and would be happy to advise.