I may have a brain but I have to say it is small, old and wrinkled! But thanks anyway!
No, I didn't vote for Trudeau either time and won't be next time!
I tend to be a minimalist at heart and try to make do with what I have or have good justification for adding to my "stuff". We have enough trouble getting gun stuff and reloading stuff here that I choose not to complicate my life by picking odd calibers to load for.
I tend to like old timey BP cartridges and quite a few of them but a) I can't afford the guns and b) brass tends to be hard and expensive to get. Consequently I would choose say .45-70 over most because a) it has some good history and b) it will do everything I need or want and can be light loaded with round ball or collar button over a few grs. of Unique or some other fast powder and be cheap to shot but still effective or loaded up to uncomfortable recoil levels with heavy bullets. .45-70 brass is more or less proprietary but there are so many .45-70's around that I douubt brass availability will ever be an issue.
For a more general purpose plinker to huning caliber and in line with the OP's intent, and because of free or very cheap brass if I was looking at even more restricted availability then it would make sense to me to go with .223, 7.62x39 or .308 simply because guns and brass will be here for the rest of my life anyway and they all have a very useful range downloaded to .22 like levels for quiet/cheap/plinking use to pretty decent levels for longer range or hunting.
beshears mentions 9mm or .38 special which are also fine and 9mm and .38 spl will also be around for a very long time but neither 9mm or .38 special is a very long range round even in a rifle. Different uses/purposes. Also in those cases I don't have guns so back to using what I have or choosing what will best suit my needs. Having said that, I'd rather have a 9mm handgun or rifle thay I could find ammunition or reloading supplies for than a .577-.450 I couldn't (zombie apocolypse/SHTF).
Everyone has somewhat different wants and needs and abilitty to buy something new so their choices will be somewhat different.
Musings from a small, wrinkled, old brain...
YMMV
Longbow