I've said it a million times already, but I'll say it again:
Trying to fix this problem by "engaging" developers is a complete waste of time. As long as consumer interest in WM remains as low as it is, there is absolutely no reason for devs to take interest. MS has been "fiddling around" trying to make app development easier and porting cheaper. While that's fine and certainly not useless, no matter how much MS optimizes dev's expenses-side, it's all for naught if the income-side doesn't provide a worthwhile ROI. WM doesn't. MS must first convince far more consumers to want WM! Then, and only then, will devs follow.
The above applies to specialty/regional apps. The types of apps WP/WM is currently most sorely missing. Apps that are already globally successful, like Google's Services or Snapchat, are another matter entirely, but I'll ignore those here.
This is not really a circular chicken-and-egg problem like everyone thinks. It matters where MS starts, and they've only ever approached it from the wrong end.
Anyway, more on topic...
MS developing/financing apps for other companies is a terrible idea. It leads to the types of apps we know all too well... lying around in a dusty corner of the store, never updated, and far less functional than the same apps on Android and iOS. That is inevitable if the company running the service doesn't care about the WM app.
Worst of all, MS taking this approach would convince almost every last app developer to drop support for WM entirely. After all, why should any company invest in making a WM app, if MS is desperate enough to shoulder the cost of developing a knock-off themselves?
Compounding the above is the fact that having your mobile app/service on iOS and Android is the only thing that really matters. The popularity of your service/app on those platforms determines a dev's success or failure. Diverting already sparse resources from those efforts, just to make a WM app, is therefore often not a rational choice to make. It's better to invest those resources into the projects that matter. That's the real problem for WM.
It's ultimately all about consumer interest and profit potential. All the people wondering more about developers rather than consumers, and how to get developers more engaged in the MS ecosystem, are asking the wrong question and missing the reality of the situation.