The shopping mall has changed quite a bit from its golden era in the 1960s, '70s, and '80s. Many centers are now built as open-air plazas rather than enclosed malls; while there will always be room for the traditional enclosed mall building (especially in areas that experience weather extremes, like Tucson's heat or Buffalo's snow), open-air structures are a growing segment in the retail landscape. Water runoff control during rain is essential in structures like these.
Shopping centers need gutters that are bigger and stronger than what you'd find on a typical residence. The roof on a shopping center will likely be a lot wider and longer than that on a house, especially when you take the overhangs into consideration, so there is more runoff speeding toward the gutter at the edge of the overhang. Also, in a house, residents tend to stay outside for a while and then go inside for a while. In a shopping center, customers are going in and out of the buildings constantly, and you don't want overflowing gutters to make that difficult.
Overhangs and Diverters
No one wants to have to cross a waterfall to get to a store. All overhangs need to have a diverter in place near the entrances to stores. These are L-shaped brackets that sit on the roof near the edge; when water hits them, it runs along the brackets to the nearest gutter. These are better to have than a plain gutter right over the entrance to the store because gutters can overflow if there is a blockage. The diverter will simply divert water to either side. If there are any gutter overflows, then, they will occur away from the doors.
Downspouts and Drainage
Similarly, commercial downspouts need to be larger and more numerous than residential. They should also be in out-of-the-way locations and point toward drainage areas that are kept free of leaves and trash (a never-ending task in a shopping center). You'll also need to inspect them frequently for signs of damage due to issues like vandalism. One other feature you may want to have is a rain-retention system so that you can use that rainwater for grey-water purposes like watering shrubbery.
As you remodel your shopping center, have a gutter and roofing company that specializes in industrial and commercial gutter installation take a look at your plans. You may find that you can work in some very sturdy gutters and rain-retention options that make your center a nice place to be even in inclement weather.