Bruce has also done 2 Minutes To Midnight, The Prisoner, Run To The Hills, and he's pretty much the only one that does Flight of Icarus anymore.
The only singer from a former band that played a lot of their former band's stuff was Rob Halford. Even on the Crucible tour he played a lot of Priest hits/deep cuts. As the solo career gets more successful you phase out the old band.
Don't think Tarja has that kind of quality under her belt. Plus there still the problem of Nightwish not being able to play most of those songs in the proper key, even with Floor.
I listened to three Babymetal tracks...sounds like Chun Li getting butt-fucked by Andrew W.K. That's not something I ever wanted to hear.