300 blackout/AAC/Whisper is a good add to what you have. Just a barrel difference from your .223/5.56s. Has advantages over the 7.62x39 in magazines and bolts. It is an accurate round and you can go from 100 grain to 240 grain in the .30 caliber. The 125 grain BTs are darn accurate.
.22LR is a good suggestion as well, but DON'T get a conversion, go with the dedicated upper or full rifle. The cleaning sucks as does the accuracy with the conversions. If you want to build one, components are failry available too. There are 2 classes...M&Ps and everytihng else which use Black Dog mags primairly. There are a LOT of options so know what you are looking for before you start down the path.
.450BM/.458 SOCOM. I prefer the .450 for a variety of reasons, but both are capable and fill the big bore desire.
One of these three would be what i would suggest.