Given that ARs are the most popular rifle design in the US, if banned it seems the obvious question is not what would I be shooting, but who would I be shooting it at?
Let's ignore such unfortunate scenarios. If the AR just didn't exist? For the AR15, I probably wouldn't bother replacing it, or I might buy a custom 1:7 twist .223 bolt gun. There's not much the AR15 does that I care about, other than being sort of recreational fun and cheap to shoot. Of course, if it weren't for the AR15 .223 would not be cheap to shoot. It's a good gun for newbies, women and children I guess. The varmints that the .223 is appropriate for, I don't hunt. As an anti-personnel cartridge it's a poor choice. Overall, I don't much care. Most of the .223 auto alternatives are inaccurate junk.
The AR10 is a bigger problem, since it's the most effective accurized battle rifle anyone has yet come up with. The closest alternative is probably an accurized M1A in a Sage chasis, whereby you'd end up with a gun that cost twice as much to be not nearly as nice. But as far as I know all the alternatives are even worse, since pretty much everyone desiring to make that style gun has settled on some variant of the AR10 as the best platform.