On the game side, the big thing would be making an M9-style pistol feel different from the current sidearm options, rather than just adding another pistol that fills the same space. Mag size, recoil control, attachment options, and how reliable it feels as a backup weapon would probably be the main pieces there.
If it were added, would you want it to sit more as a steady, general-purpose sidearm, or something that stands out with attachment options like an optic/rail setup?
Also, where would you want it to fit compared to the current pistols: easier to control with less punch, or a stronger backup with a little more recoil/tradeoff?