We're very excited about the Windows platform!
- For one thing, there are many MIDI targets will run on it, including many DAWs and plugins.
- For another, in our opinion, .Net has been an exciting platform/framework since launch, and C# is a great language.
It would be a complete rewrite of MIDI Designer, which is an exciting prospect, but a huge one that will require time/money.
For right now, we're not announcing the arrival of MD on any new platforms, but if all goes well, we'll have something to announce in 2017 (probably not ready until 2018). This also depends on how Windows marketshare grows, and what kind of MIDI support they provide for .Net.
For now, iPad is the best way to use MD.
Thanks for your question!
Also see this article from 2014 which goes over this topic in greater depth.