Weather sealing, strong enough construction allowing for mounting on the sides, windows on front (cab side) and rear (tailgate side), and a positive pressure vent to minimize dust intrusion.
If you have any plans of sleeping inside (or even storing a fridge, etc), add an insulated canopy to that list and interior opening to the list; if you're purely using it for storage, opening side walls/doors are a nice addition.
FWIW, we had an Alu-Cab Explorer canopy on the better half's previous long bed Tacoma (used just for storage, although I did spend one night in it) and an AT Overland Atlas canopy/camper on my previous double cab Tundra.
The Tundra/AT Atlas was awesome because the interior was set up for "living" space (decked drawers, bedrug on top of those, fridge, seating space, Truma heater, toilet, battery/inverter area, etc). The Tacoma had decked drawers and was almost always used for hauling plants, so having the side opening doors was super convenient to grab stuff and for hosing it out