The BTR-90 was essentially a BTR-82 improved body enlarged and with a BMP-3 turret fitted.
And there have been other interesting experiments with the BTR where gun turrets have been fitted including 85mm turrets and even 125mm Sprut turrets...
But really for Russia using older vehicles is going to make less sense as they are withdrawn.
Vehicles like the Hosta make sense because they have enormous numbers of 2S1 vehicles and parts and support equipment so putting a 120mm gun/mortar on the chassis makes sense in terms of eliminating a calibre (122mm) while keeping a capacity... the 120mm shell reaches about 13km which is comparable to the 122mm guns 15km range.
Plus it can use mortar shells as well... plus the guided 120mm munitions and 122mm guided munitions too.
But what is a bit crazy is various factions and countries have mounted BMP-1 turrets and also just the 73mm gun itself. Cuba turned the Grom 73mm gun into a towed anti tank gun. Namibia have mounted the BMP-1 turret onto two Chinese 6x6 wheeled APC and onto a home-grown MRAP. And some crazy factions, like those in Libya, Syria, Yemen, etc have just mounted the guns and turrets onto pick up trucks which I think would be extremely top heavy for a pick up. See examples below. (The Cuban 73mm grom towed anti tank gun is the Cuban thread)
The SPG-9 is a potent weapon and its reduced weight because it is a recoilless weapon instead of a gun. You get lighter cheaper ammo if you use it in the 73mm gun of the BMP-1 and against many targets it is still a potent weapon.
With a gun mount then the back blast of the SPG-9 becomes less desirable and its lighter weight and low recoil don't matter, while the 73mm guns improved performance makes it a better weapon.
Again it really depends on production capacity and ammo left over... I would say Russia would do best to sell off its 73mm guns, but as they are upgrading their SPG-9s maybe ammo adaptations would allow them to use old stock 73mm ammo in the upgraded recoilless rifles... the HE models don't need extreme accuracy against area targets...
The existing rocket ammo could be upgraded with ramjet powered missiles that have a flatter flight trajectory and shorter time to target reducing the effects of gravity and sidewinds leading to higher kinetic speeds and improved performance too.
73mm is a good calibre because it makes the ammo and weapon rather light and mobile, but if you want improved ramjet performance you could go for larger calibres easily enough.
Few modern forces will operate with no vehicles at all... even helicopter delivered forces can have vehicles to improve the volume of equipment and weapons they can take with them.