It would be really great to see .45acp firearms added, from a 1911 (stashed level as a WW2 style, modded version with rail and suppressor mount) or FNX45 as pistols, to a UMP45 SMG, or even a Thompson (stashed model could be stick mags only, purchased one could take sticks and drum mags) as SMGs. Recommended as a security level 3 caliber too. Would likely hit hard because of the denser round, but wouldn't be the best for penetration, and would lend itself especially well to being suppressed.
#New Caliber suggestion - .45ACP
mild ledge
ocean tide
It is unlikely we will be getting any new calibres (until the 1.0 release)