Cross-project interaction has been a fairly common topic proposed by other users (feel free to check them)
However this begs the question if implementing cross-project features the would bypass free account limitations? and might this be delaying such ideas?
Regardless, I would propose more simpler/short termed features:
- Syncing list of keywords, categories and groups between projects (since import/export tools already exist)
- Mother/Hub project. essentially have a project that collects/links knowledge items and (in the future) references without physically importing them. (Automatic Sync could be customizable based on user's keywords or ref. parameters)