I admit I'm the least informed on this, so I can only speak from an arcade fan/enthusiast prospective.
Neo Geo might have one of the most memorable library of games of all time.
Just think of Metal Slug, Samurai Shodown, Puzzle Bobble and King of Fighters. The latter even have new installments on current hardware architectures (PC, PS4) while the others have truckloads of remake, due to their sheer quality, replay value and, last but not least, nostalgia factor. They have a place in pop culture, I think everyone who ever set their foot in an arcade knows what Metal Slug is.
So I'm not that amazed at how big the community is and the request it had for a flash cart (which we now have, thanks to NeoSD).
On the other side, we have Capcom, which is a giant still today with IPs like Street Fighter still going strong and has its roots in the arcade hardware back in the late 80s and early 90s onwards.
If you ask anyone about Street Fighter, you have a very high chance he knows what it's about.
Not just that, take a look at the game catalog on CPS1, 1.5 and 2:
http://system16.com/hardware.php?id=793 (CPS1)
http://system16.com/hardware.php?id=794 (CPS1.5)
http://system16.com/hardware.php?id=795 (CPS2)
1941 series
Final Fight
Ghouls'n'Ghosts
Street Fighter series (plus all the various crossovers and marvel fighters)
Cadillacs and Dinosaurs
The Punisher
These, just to name a few of the most iconic games ever made.
IMHO, a CPS1+1.5+2 multicart would sell really well, without mentioning the possibility of an FPGA based motherboard with modern outputs like HDMI (or, to avoid royalties, DVI, you can buy a cheap DVI-HDMI adapter on amazon for a few bucks) which would enable literally EVERYONE to play those classic on their modern sets, which means it would sell like hot cakes even to those who never gave a shit about arcade hardware, just to play the classics on their TV sets without any fuss.
This, in my opinion, would be a highly marketable (and desirable) product, aimed to both arcade enthusiasts and the average joe who wants to play those old games without the mess that is MAME and emulators.
EDIT: just to make it clear... I'm not asking for a CPS2 multicart which is able to play CPS1 games.
I'm saying... just make a single multicart with the connectors to use it on either a CPS1 or CPS2 motherboard, so that you don't have to buy separate multis. You put the multi on a CPS1, you play CPS1 games. You put it on a CPS2, you play CPS2 (maybe CPS1.5?) games, instead of making two separate carts.