My suggestion is so simple that it may have been suggested before in the past. If so, apologies for the redundancy.
When I type in a term that I'm looking for, and MultiTerm produces no results, in many cases I would like to add that term to the termbase. When I press F3, I have to "start over" and re-enter the source term. It would be nice if MultiTerm would automatically insert the term I just entered into the source term field, so that all I have to worry about is the target term.