Sig-Brace all the things.... The non-SBR SBR category seems to be one of the few that's growing in the market.
Skirting the NFA is getting more and more popular.
The 14" Mossberg 500/590 with the birds-head grip that is neither SBS or AOW, which I put together about two years ago... is now a production item from Mossberg, and perpetually sold out.
Optics that people can actually afford, like Primary Arms and Vortex seem to still be doing strong. (IIRC, I know Monkeyleg's said the buy-in for them to become a dealer was exorbitantly high...)
Cerakoated Glocks in FDE, desert tan etc. with red anodized parts, and lots of extra milling on the slides like from Zev and their ilk seem to be popular. And the various YouTube "Bearded millennial/GenX veteran channel guys" seem to be showing them off.
Bullpups like the AUG and even moreso the Tavor SAR and X95 are still hot sellers. Prices have not fallen like the AR glut has.
High capacity futuristic shotguns seem to be doing well. Be they converted Saigas or Vepr's, the USTS-15 from Turkey, the Keltec KSG, the SRM 12 with the four rotating mag tubes.
Precision bolt action rifles seem to still be hot, be it Ruger's new precision rifle, or just aftermarket skeletonized stocks for them.
They're still damned expensive, like $1200-1500, but HK MP5/94 clones from Turkey seem to be on the rise.
I've been seeing a LOT of videos and pictures of hog hunts etc. from those ATN $1200-ish "digital night vision" scopes on various gun boards and Youtube. And while most of this ATN digital line that's under $2000 has no true electro-optic NV, or actual FLIR-Thermal, I don't see people making fun of it in the comments either.
Medium to high-end PCP bullpup airguns for pest elimination as a "hobby" of sorts, or for cheap quiet shooting in suburban backyards seems to be catching on.
As part of the Sig-Brace/arm-brace trend, 9mm AR pistol caliber carbines are becoming popular.
I have no idea how someone would be able to legally beat Ruger to the market, unless it's old enough no patents exist, but if someone could produce a Mini-14 in stainless, in the "GB Factory Folder" (pretty much what the A-Team had on their TV show) they'd sell a boatload of them, and be able to do it for over (but not too much over) $1000.
I expect for there to be a minor run on HiPoint 995 carbines when Hightower Armory starts shipping the bullpup conversion stock sometime this summer.
If someone could make a reliable .22 LR conversion slide and magazine for the brazillion S&W Shields in 9mm and .380 that have been sold in the past year or two, they'd sell a ton of them IMO.