This was one of my thoughts. Pick up an AR in 556, and then build an AR pistol (I'd just put in the pins, and have a local smith actually put the thing together from the parts) in 40, since i reload for it. And honestly, I've been wanting one similar to the one Badbob has since I saw the picture of itYou're covered in the pistol department fairly well. The week spot IMO is long gun in popular calibers.
The AR platform allows easy transition to whatever your needs dictate.
I would suggest a basic AR15 in 556 to start with. Then maybe a hunting rifle In whatever platform you choose in a heavier caliber (308, 30-06, etc).
With the political climate and frequent "crisis" buying frenzies having options in popular calibers just makes sense to me.
just saw y your second post.
Adding a PCC in 357 or 40 might be another option to consider once you fill the gap in rifle caliber.