I don't see technology as having necessarilly stagnated as it seems to simply have hit the limit for what is possible in several areas.
We're used to a world where Moore's Law (Computer speeds doubling every 18 or so months) prevails. However even Intel's engineers say eventually we'll hit the limit. Current transistors require more and more cooling as the power is fed through smaller and smaller areas.
Material engineering (most of battletech's basis) isn't a limitless thing, eventuallyyou simply hit limits. You can't build stronger armor, or you can't forcemore energy through a laser within a certain amount of time. Most of battletech's systems have compromises in order to reach those limits as they are.
I think more of the technology in TRO-3150 should be buffer or debuffer technology (think targeting computers or stealth armor) rather than actual weapons systems.
There's no need for better heat control, in the game itself, unless it comes with serious compromises in order to maintain balance. For example, a triple heat sink that can only work for a certain amount of turns before it goes down to being a single heat sink. Or perhaps is explosive.
For Autocannons, it could be a simple reordering of them into more sensible lines.
Long Caliber, Medium Caliber, and Short caliber this affects the range and the tonnage of the weapons. (replacing Light and HV cannons with an easier to understand system for new players) LBX should be merged wtih the other specialty ammos available to ACs, and RAC and UAC should be merged into a multicannon system with various multiple fire being available for spending more tonnage.
So for instance a 4 barrel Long Caliber autoCannon 10 coudl fire up to 4 rounds in a turn, and would have the longest range available to the AC-10s. If firing singly it coudl fire specialty rounds. A single barrel Short caliber AC-10 coudl fire a single round only, and would have the shortest range available for an AC-10
Various equipment would boost the performance, for example a recoil stabilizer would give better cluster performance to guns firing multiple rounds.