I also think there will be an interest in the "also-ran" oddities.
While it is very difficult to predict what is desirable or collectible in the future, many times, what eventually becomes collectible is that which is the first of something, that which is of good quality (does not have to be great), that which is scarce, and that which was longed for before adulthood.
It's like asking which cars produced today will be collectible. Who knows?
Truly, if we are talking 50 years from now, what kind of exposure do kids have with firearms and what do kids dream of having? Unless they have a connection and good memories with a parent taking them to the range, a lot of exposure has to do with video games.
Hot take here, but without political regulation, the generic AR-15, G19, and other firearms found literally everywhere will not be a collectible by and large. There are way too many of them out there. The "Honda Civic comparison" applies here - just too many produced to be collectible.
I could see something like a Bushmaster ACR, Beretta ARX, or other AR-15 competitors being collectible, though. These are proprietary and lower volume - kind of like how the Steyr AUG is desirable now for being different.
I'd also agree that the Beretta 92 is "taking the place" so to speak of the 1911. Let's say, Beretta discontinued those M9A3's or say Wilson Combat went out of business (not wishing it, just saying IF), then those guns would be highly sought after (as in the specialty ones, not the everyday 92fs or M9).
DEagles and S&W 460/500's will always be admired, sought, saved for, purchased, fired, and then sold (haha) because they are the biggest, baddest, and "best."
Almost anything 50BMG.
I would guess the FN PS90 just due to how different it is.
I estimate that desirable, high end shotguns would be good. However, most don't want a stock custom fit for someone else. .
Anschutz Fortner rifles. The 2072 Olympic Biathlon will probably still compete with these "relics." Energy rifles shoot way too flat and are unaffected by the wind (haha).