There are already a few options available in EmEditor:
1) have you had a chance to look at the autocomplete plugin?
2) have you had a chance to look at the wordcomplete plugin?
One or both of these may come close enough to what you are looking for. If not, you might still be able to design something like this yourself using regular EmEditor macros, specifically with the “popup” menu feature therein.
3) have you had a chance to see if macros can do this for you?
I’ve already implemented a version of “dynamic snippets” that pretty much does exactly what you describe (unless I misunderstood you). The user types in a word and then presses tab key (or other shortcut key), and a snippet comes up based on that trigger word. If there is more than one snippet that has that trigger word as a substring, a menu appears to let the user pick which one they want to insert.
For more details of what it looks like, watch the videos at:
http://www.emeditor.com/modules/newbb/viewtopic.php?topic_id=965&forum=2