If we're talking about a rifle that is going to be produced in peacetime, warehoused and then distributed to conscripts / militiamen, then the AK has been designed specifically for this.
Goverment-issue "Citizen Rifle"
What I understand under "Citizen Rifle" is a rifle that would be considered "unalienable", a rifle that every US citizen would have the right to possess so as to respect the Second Amendment - and thus would enable to get rid of THE REST OF THEM, of course.
True citizens are not letting any government dictate them how much force they ought to have, so let's imagine a rifle from the government's point of view, then.
Any strong government dislikes citizens too powerful to overthrow it. This means the weapons should not be able to provide its user with a high rate of fire, for instance. e.g. Exit semi-auto.
Too light a round is not good as it enables each citizen to carry more of them. A heavy round means less ammo carried, especially in the hazardous logistics of a civil war / insurrection etc. Similarly, it must not have a military caliber, so insurgents won't be able to use captured ammo. Anything less than a .270 would not be accepted, if only because of the hunters.
So we down to, basically, a hunting rifle.
I come from France, and I can tell you that's just how much rifle you can buy without asking for a special authorisation from the county. But a lot of people have such authorisation, and a lot of french shooters have AR-15. But these are to be in the custody of the police in case of a state of emergency - and the hunting rifles, less so.
Still, the hunting rifle has its niche, in harassing techniques, sniping etc., but it requires trained shooters. Hence, few insurgents would be able to concentrate enough firepower with it to pose an immediate threat to the State.
Franc-Tireur Rifle
In 1870 the french army was meant to be supplemented by patriotic shooters, who would form a sort of milita, armed with their own weapons. These groups were called "Francs-Tireurs" and had a limited effect on the war, mainly due to the poor discipline of some of them. Nevertheless the Prussians went completely hysterical over them, and it's with the "Francs-Tireurs" in mind that the Laws of War regarding partisans (necessity to have a recognition symbol etc.) were made.
If we we to devise a "Citizen Rifle" that would be used by a "Francs-Tireurs" system in the US, then it could be a different rifle than the hunting rifle above.
In such a system, the Franc-Tireur has to purchase his own weapon and uniform, and practicaly, he has to bring his own ammo with him. A lot of Franc-Tireur had non-standard weapons and calibers, just like we would.
In this "voluntarily conscript" system, we would go closer to the AK again, but without the logistics of vast conscript armies like the soviet one, meaning "spray and pray" as well as suppressive fire techniques would not be appropriate. Given the relatively poor training and fitness of the Francs-Tireurs (when compared to a soldier), their rules of engagement should not be the same, especially regarding the range of engagement.
Hence, we could indeed contemplate the use of the .308 cartridge, which has a longer range than .223. .308 is already popular as a hunting caliber, so it is generally available, and occasional resupply could be done by the regular army as well.
In our time, scopes are no longer a luxury, and should evidently be installed on such a rifle, because of its long-range use.
The Franc-Tireur would then be a sort of marksman (not unlike the french "Chasseur" of the same 1870-1871 war) capabale of supporting regular troops or to be used in delaying the enemy, harassment, and general partisan warfare.
Weapons in this category would be the Zastava M76, scoped M14, MAS 49/56 etc.
My personal note would be to make it in a bullpup configuration, because urban combat is going to be the main form of combat (with more than half of the human population living in cities), but it requires additional training, so a traditional configuration would be better suited for all.
ADD to EDIT : Because of the affordability issue, it would be Saiga .308 with a fixed-magnification scope.