Hi Chris,
Here are my thoughts:
1. I'd like to see 2D physics integrated into the Flash & AIR runtimes if it would provide significantly faster performance than using a 3rd party library.
2. My preference would be an implementation of Box2D. I know Nape is an awesome physics engine but I'd rather Flash natively supported Box2D because of its popularity across other languages.
3. I'd only really be interested in physics being integrated into the runtime if it were to be properly maintained. For example, if there are updates to Box2D then I'd expect to see the updates to the ActionScript API also.
4. I would probably prefer to see physics integrated within the runtime rather than a separate ANE.
5. I use AIR primarily these days and target mobile. However, it's important that the physics engine is available across both Flash Player and AIR.
6. Some features that I'd really want to see over and above the absolute basic 2D physics engine stuff:
i. Ghost Vertices via b2ChainShape. (http://www.iforce2d.net/b2dtut/ghost-vertices)
ii. Collision Filtering via categories and bit masks. (http://www.iforce2d.net/b2dtut/collision-filtering)
iii. One-way walls.
iv. Sensors (http://www.iforce2d.net/b2dtut/sensors)
v. Joints - including revolute, distance, line, weld, pulley, friction, gear etc
Thanks.