IF I was going to build a combination weapon the shotgun tube would be 28 guage.This under appreciated shell will print as well as a 20 or 12 gauge with far less recoil.
The rifle cartridge would be the .30 US aka .30-40 Krag. This round falls in the lower pressure group but can load the potent 220 grain roundnose ( it's original loading.) for moose & bear.
The .30 Krag is also well known for 'squib load' friendly loading.
The problem, is that today's gunmakers will build actions around cartridges. One of the reasons the litle C 6 went out of production was in fact receiver issues with the .22 Hornet.
Buyers will want familiar cartridges like the .308 and 20-12 gauge. And even if the industry builds what they ask for, a fickle buyer will fail to buy in profitable numbers.