Can you change the default Apple Watch watch?

I'm planning to make an watch app for the Apple Watch.

Is there any way of replacing the default home screen (or watch) of the device to be a custom watch?

Assuming there is none, the next best way would be to make a WatchKit App complete with with a Glance?

Does the apple watch come with a power saving mode by default where it will turn off the screen/running app reverting to it's home screen after a certain period of inactivity?