Senior
Let several people edit one document at once, see each other's changes live, and merge work done offline.
Build the editing service behind a team docs product. Anyone on a team can start a document, and several people can type in it at the same time, even in the same paragraph. Each person sees the others' typing within a moment, with a named cursor showing where each collaborator is working.
People sometimes keep writing on a train with no signal, and expect their changes to merge in when they are back online without wiping out anyone else's. Documents stay in use for years and build up long edit histories, but should still open quickly.