I'll toss out a few, don't put anything in or near the cab headliner by the door, I've broken off more knobs, switches and controls with coat hoods and pocket catching on things as I climb out over they years its not funny.
If its heavy equipment, have the cab door lock open somehow, a lot of stuff is only to be run with the door shut, there's no way to latch it open.
I want the entire cab to come off, not in days or hours with a lot of help, but in less than one hour with one person and a crane I can take it off, take it to the shop and troubleshoot switch issues, seat problems or whatever is inside it, while on the ground, not feet up in the air.
I'd also like to have a complete contained ac unit, mount the compressor on it, have it driven off a hydraulic motor and if there's an issue, swap out the entire unit and go or take the entire unit off and take it into the shop to be worked on, that way everything is contained within, somewhat like a fridge or freezer is done.
I'll add to the comment of flat glass, whoever invented curved glass should be shot.
On the cab, have the glass held in with rubber gasket material, but have the entire window frame unbolt and be taken out of the cab, this does two things, first I can take the frame, gasket and window out as a unit to be taken into the shop to replace the glass, so I don't have to take the machine in or put the glass in the gasket myself or have a service call to have it done, then have a hook on the outside of the frame so I can use a crane to help put the whole assembly back on the cab, without a dozen people helping me install the window or glass, doors can be taken off, but the main cab windows usually can't.
No plastic inside the cab, I could care less about looks for my heavy equipment, bolts are the only fastener to be used in putting anything in the cab or putting the cab together, no clips, screws or fasteners that require anything but a standard set of wrenches. Headliners should be bolted in place, not glued on, you also need to come up with a much better design to deaden sound and insulate the cab than is available today, most stuff used today is junk and won't stand up the dirt and vibration over time.
Air is the only thing to use for seat suspension, if you have to use an air pump for the seat, don't mount it under the suspension like some have done in the past.
Put whatever you do on actual jobsites for testing before ever putting it into production, tell whoever is testing them to go tear them up, if they can, its nothing like will be done to it in normal use and abuse on many jobsites.