As Pat said, a Garmin 660 or 665 will drop right in, but not interface with the bikeís ScrollWheel. This mostly allows you to zoom the map and scroll through menus, plus it reads some bike information like fuel range.
A BMW Navigator IV, V, or VI will fit and interface with the bike, but each requires a different dash surround as they have progressively larger screens. Those cost around $50.
Your 2015 likely has the dash surround for the Nav IV. The others will fit, but the Nav V and VI will have some of the screen covered.
The Nav V screen is larger and easier to read than the Nav IV, but the Nav VI screen is way better for size, clarity, and sunlight readability. It really is a nice upgrade.
You can pick up 660, 665, IV, V, and sometimes VI units used for decent prices. And some forums, like K1600forum.com
have good vendor discounts on new Nav units.