>>63964974
> maybe fit the magazine in the grip to make it more light and compact
Maybe something like the Russian SR-2M Veresk? Gas-operated, in a proprietary 9x25mm cartridge - a bit heavier than Tokarev, and with comparable or slightly slower muzzle velocity (depending on the specific Tokarev loading).
The Russians, unlike FN & HK, didn't go for the small-caliber PDW cartridge and just decided to make overpressure 9mm with penetrator cores (they also have such loadings for regular 9x19mm)

But overall, this is indeed an underexplored area - submachine guns in calibers spicier than the typical pistol calibers. SMGs on the whole are held back by pistols - people don't want to be introduce a whole new caliber into their arsenal, so SMGs are developed for the same caliber pistols are, but the larger form factor of SMGs allows you to use other operating system (like gas operation, or various forms of delayed blowback) and handle way more power than a pistol could.
There were some attempts at making 10mm Auto SMGs, like the MP5/10, but those didn't go anywhere. There's also the M2 Carbine, I guess (although some argue for .30 Carbine being considered a proto-intermediate cartridge), but that's not very controllable - one could probably design a modern .30 Carbine SMG that's a lot nicer to shoot.

But with .300BLK SBRs being popular these days, I guess this is an area of development that just doesn't attract much interest.