I think this would fir in with the Mobile API, although I could see it as part of the core offering.

Right now there isn't a great way to divide up a project to be able to share code between WP7, Android and iOS.

Many developers, myself included, have been copying project files that end up having the same source files and maintaining a large number of different projects for each platform.

Even if there was a tool that auto-regenerated this project files or something of that nature, it would be better than everyone rolling their own solution to this problem.

If this issue is addressed, and by addressed I mean there is an easy to explain understandable, solid way to do cross platform mobile development with a shareable code base, I think it will go a long way to MonoTouch / Mono for Android adoption.

I'm not looking for an integrated UI solution here, but a way to bridge the common parts so that I don't have project hell.

Create a basic layout engine (perhaps using a subset of XAML) that then compiles to UI for Android, iPhone and WP7.

It would require developers to code to the least common dominator, however it would give a way to build UI once and run it on all three platforms.

I know Xamarin's model is "build native" but sometimes this just isn't an option due to the sheer number of UI pages that must be created, and sometimes just having something that works but doesn't look native is an acceptable trade-off.

It should support the most common controls that already exist on all platforms:
- Labels
- Buttons
- Text boxes
- Toggle switches (checkboxes)
- Images
- Sliders
- etc.
-