I think that, for practical purposes, the Scorpion 90 maxed out that chassis; yes you could get a bigger limited traverse gun but I'm not sure you'd be able to do much the the 90mm couldn't and losing a rotating turret is not a small matter. The Russian 100mm 2A70, as fitted on the BMP-3, might be about the same size/recoil as the 90mm, but even better, with a good sized shell for HE work and the missile option for AT work.
I also think that the S-tank style would be an interesting approach to maxing out the capability of a light AFV capable of lift via helicopter (CH53/CH47 class) and used for airdrop. It allows for a good combination of protection and firepower in a small footprint and the ease of using an automatic loader, along with S Tank style controls, would let you drop the crew to two if you're willing to accept a commander/gunner combination (not desirable, but decreasing the volume under armor is a big deal when trying to protect a lightweight vehicle).
Ref the M1128 Stryker Mobile Gun System, and these kinds of systems in general, they can't be used as tanks but they can do infantry support, following the infantry and then shelling defending strongpoints, and they can do well in the defense (as the US WWII Tank Destroyers, as well as the German Panzerjäger's, e.g., the Marder series, showed). My thoughts on the M1128 are somewhat similar to my thoughts on the Scorpion: there was no reason to try and put a high velocity 105mm on it, maxing out the chassis, when a modern turreted 75mm or 90mm gun, backed up with a couple TOW missiles in a box, could have done all the same tasks with less weight and more available ammuntion.