A PDW in rifle caliber. AR-based PDW, a .300 BLK AR pistol with an 8.5" barrel (or thereabouts). Add a "folding stock adapter" and a brace, and you've got a pretty compact PDW that folds to under 20". It won't replace everything, but it does a lot of jobs reasonably well. A handgun is far more concealable, but underpowered. An AR is a bit more powerful (or to be more precise, has a longer effective range), but far less concealable.
Other alternatives would be something built around a 7.62x39 platform, with a folding stock. Krinkov, etc.
Just my HO.
ETA: it really depends on what you want the weapon to do, of course. Coming from a different perspective, I'd choose a suppressed SA .22lr pistol, like one of the threaded Ruger Mk2/3 types, or an equivalent Browning Buckmark. .22lr makes a great suppressed survival weapon. Nearly silent, and you can carry tons of ammo.