The biggest limitation to innovation of a handgun (or really any firearm for that matter) is the ammunition. All designs basically have to accomplish the same three things: insert a cartridge into the chamber, fire a projectile, and remove the spent shell casing from the chamber so that another can be inserted and the cycle started over again. Of those three things, firing a projectile is the defining characteristic that makes something a firearm so the insertion of a cartridge and removal of a spent casing are really the only two points upon which the design of a firearm can be innovated.
Over the last 150 years or so (since the dawn of the self-contained cartridge) those two functions have been accomplished just about every which way you can accomplish them be it through manual means (revolvers, single-shot/multi barrel guns, and manual repeaters like bolt, pump, and lever actions) or through harnessing either the gun's recoil or gas (full and semi-automatics). For about the last 100 years, the majority of the innovation has come in the arena of automatic weapons, and even that hasn't really changed all that much over the past 50 years or so.
Look, for example, at the popular semi-auto handguns of WWII and earlier as opposed to those of today. Nearly all use either the Browning short-recoil method of operation (1911, Hi-Power, TT-33, Glock, Sig), locking-block recoil operation (Walther P38, Beretta M92), or unlocked blowback operation (Walther PP, Beretta M1934, Colt M1903, Mauser HsC, Sig P232, Hi-Point, Astra 400). Trigger actions are either single-action (1911, Hi-Power, Springfield XD), Double Action (Walther P38, Sig, CZ-75), or some combination of the two (Glock). Just about every successful semi-auto handgun since WWII feeds it ammunition from a detatchable box magazine that is inserted through the grip and is of either single-column or staggered column. Even the ammunition isn't all that different, both 9mm and .45 ACP, cartridges introduced in the first decade of the 20th century, are still among the most popular cartridges available.