Mainstage is not available for Windows but there are some alternatives that runs on Windows with similar functionality. The best Windows alternative is Cantabile. It's not free, so if you're looking for a free alternative, you could try Carla. If that doesn't suit you, our users have ranked eight alternatives to Mainstage and five of them are available for Windows so hopefully you can find a suitable replacement. Other interesting Windows alternatives to Mainstage are Blue Cat's PatchWork (Paid), Gig Performer (Paid) and Camelot Pro (Paid).
Carla is an audio plugin host, with support for many audio drivers and plugin formats. It has some nice features like automation of parameters via MIDI CC (and send output back as MIDI too) and full OSC control.