You can buy a Phase One back in the following mounts: Mamiya/Phase, Contax, Hassy H, or Hassy V. The user cannot change the mount of the back. You can exchange your mount for a fee (one free change is included if you bought the Value Added warranty).
Fortunately unlike other systems (e.g. Hassy H3D/H4D/H5D) you can use any compatible Phase/Mamiya body with any back - no need for a trip to the factory to have them matched/locked. You can also use the RZ Pro IID without cables. That body has a very different feel than the AF/DF. It has a rotating back so you can shoot verticals without turning the camera, a waist level viewfinder option (you can also use a standard viewfinder), a set of lenses known for their unique look, a bellows/rail system for focusing. It's heavier and larger to be sure, but as far as a "different body as a backup and also as different feeling" - the RZ Pro IID is perfect.
You can also explore tech cameras, nearly any of which are fully compatible with your P40+. You have a battery on board your back so to use it on a tech camera all you need is the appropriate adapter plate to get it physically mounted and a sync cable between the lens and back (probably included when you bought your back). These cameras are best for landscape/architecture/interiors but can also be used for street photography.
You can also try a view camera. They work best in the studio for still life, product, and other static shooting but can be used for anything. Older view cameras are generally too sloppy in their movements to be used with a digital back (with some exceptions and depending on your needs/applications) but any of the recent view cameras designed for digital backs work great. The availability of huge ranges of movements is their biggest appeal, as well as the interesting look you can get, especially from legacy view camera lenses which, while not the sharpest lenses for digital, do provide interesting look/feel/color.
To summarize your options:
- another Phase One DF body
- Mamiya branded DF body
- Phase One AF body
- Mamiya AFD3
- Mamiya AFD2
- Mamiya AFD1
- Mamiya RZ Pro IID
- Any tech camera
- Any view camera
Not a bad array of choices!