As you already use a Leica M you won't be worried by accessory viewfinders, so for lens quality and interchangability, extra image size, overall camera size and handling, the Mamiya 7 ticks as many boxes as there are.

If you have a strong assistant, and want to go bigger to 6x9, the Fuji 690 and GW690 are both relatively cheap, but they are heavy and gigantic (really big), but they are rangefinders, and don't need accessory viewfinders.