Perforce is a little bit different -- A source control package used by a lot of games studios - http://www.perforce.com - and we have been toying with the idea of integrating tank in perforce.
Tank Apps could for example be custom publish dialogs where a user can select a bunch of notes in Shotgun that are being addressed as part of a particular commit. Or simply deeper metadata sharing between things that go into perforce and things that are tracked in Shotgun.
I am sure there are many more (and better!) ideas -- let us know if you have something you would love to see being built!