You can just get a Ciener kit if you want to shoot .22LR with your AR.
Based on the avaliability of the ammunition argument, I would suggest 7.62x39 but it seems more practical to just get a SKS or an AK when you weigh the costs and benefits.
For another AR upper, I really like the 6.8SPC and .458 SOCOM.
There are quite a few different loads avaliable for them and they are both much more impressive than the standard 5.56mm cartridge.
Avaliability isn't an issue to me since I reload but the gist of it is that most ar variants or custom uppers are specialty builds for specific tasks or purposes.
What do you want to do with your rifle? That will determine what your cartridge requirements would be.
If you just want to plink, .223 is suitable and is accurate out quite a ways. If you want more range, you can get a match grade full length barrel etc.
If you like precision and have experience with reloading or wildcatting you could go with something like 6x45 or another bench-rest oriented cartridge.
If you want a little more performance for hunting or what have you then 6.5 grendel or 6.8 SPC are good candidates.
If you want to hunt large game with a big-bore AR the .450 Bushmaster, .458 SOCOM and .50 Beowulf come to mind.