In my opinion, collecting guns is so varied that there is no single answer. Do you want a basic utilitarian collection? A shotgun, centerfire rifle, rimfire rifle and a handgun will cover that.
However, there are fr more options. At one time, i owned only guns that were unusual for some reason. Either they were uncommon models or had some feature that made them distinctive. Probably my most unusual was a FP-45 Liberator.
You can collect every variation of a favorite firearm. Just picking the common Remington 700, there are the ADL, BDL, CDL, Safari, SPS, Tactical, R-5 and I'm sure more variations. Start figuring in caliber variations, special editions, wildcats, etc., you could have a very large number of remington 700 rifles, none of them identical.
Others opt for a collection that features a good example of every major action type. Revolver and autoloader for handguns, bolt action, falling block, autoloader, break-top lever action, etc for rifles. Single shot, pump and autoloader for shotguns. There are more types, even.
Other people collect only certain action type from certain manufacturers. I have a friend who collect only Winchester lever actions.
I don't have a "collection" anymore. I have a bunch of guns that fill roles, whether that is defensive or target shooting and a lot of stuff in between. A proper "collection:, though, should have a common theme. But there's absolutely nothing wrong with just having some guns you like or have use for.