The .40 is thought of by many as the answer to the question that no one asked.
With 9mm offering such a huge range of ammo types, you can literally go from 115-147gr, truncated cone, FMJ, HP, bonded, NATO spec, +P, +P+...you name it, they make it. It can literally be whatever you want. They even make freak, ultra light, ultra high velocity rounds that by what I've seen, can defeat light body armor.
https://www.midwayusa.com/product/1...-fragmenting-hollow-point-lead-free-box-of-20
https://www.youtube.com/watch?v=Nr6h44Pu4sM&t=298s
With all of this ammo on the market, and 9mm being not only cheaper range time, and easier to control, I do not personally see a reason to have any other pistol caliber at this point. You also get 16+ rounds in a full size pistol, my XDm holds 19 +1.
Build a 9mm AR, pick up an UZI, Beretta Storm, or CZ Scorpion and you get a great pistol/carbine combo that will all eat the same ammo.