If used right I think the show would have been better for it. But then there's the obvious point she probably wouldn't have been used right.
I think it's pretty clear they were setting up for a "Power Corrupts!" arc for her. Assuming it wasn't done as whimsically and fly-by-nightish as "Fury" it probably could have worked.
Maybe if they worked it such that Seven was immune to her powers, they could have set up a Tudor-esque conflict between Janeway's "daughters;" it might have had interesting potential.