For bigger tires, one issue is the width of the wheels.
The OEM steel wheel is 16x6.0 and the alloy is 16x6.5. While there are aftermarket 16" wheels (Vision Warrior 375 and Valor), they don't seem to be any wider. The G-Wagen Atik and Ashtaroth 16" wheels may be wider but may not have the higher load rating of the Sprinter (most G-Wagen load rating specified at 1100 kg which is lower than the Sprinter's).
In the 17" wheel size, the MotoMetal MO970 is 17x8, 2800 lb load rating, and the recently released UltraWheels Toil is 17x8, 3500 lb load rating. Need to check the offsets and clearances. A couple of tires in different sizes have the right load ratings.
Note: PNWSquid had custom 17x8 wheels made, with minimal offset (wider track), and 265/70/17 tires (larger diameter). May want to inquire as to the pluses and minuses of his first 235 width tires and switch to 265 wide tires (offroaders in mud/snow? like narrower tires to bite through into hard soil while in sandy beach conditions, wider tires spread out the load?). How about handling in crosswinds?
In the 18" wheel size, it is more difficult to find a load rated wheel. While the G-Wagen wheels are popular, the load rating may not be as high as Sprinter. Also, proper load rated tires in 18" sizes will be the larger tires.
Note: On the original 16" wheels, folks have switched from original 225/75/16 to 245/75/16, so the tire diameter changes from about 29.5" or so to 30.5". The latter 30.5" diameter makes the speedometer read more accurately while odometer would read less. Reports are positive re larger size tires improving handling, possibly from greater load rating, wider tire tread, ???
Larger diameters also may mean less oomph from standstill but probably not noticeable unless even more than 1" in diameter.