Eh... The weight system is part of Killing Floor's DNA just as much as the absence of crosshair or the trader tbh.
I'm the first to praise change, even the most ground-shaking ones, but I believe that's something that shouldn't move. Mostly because "if it works, don't fix it"
Mind you, it is also way fairer in KF2 than it was in the first game. It wasn't uncommon to have to rely on your 9mm to survive in KF1 if you picked some top-tier weapons (the L.A.W. for the demo or the M99 for the sharpie mostly). Now, I believe you can ALWAYS have a sidearm by the late waves (7-10)
Now sure, you cannot use EVERY combo of two weapons as you mentioned...But it's actually part of the balancing factor too. True, the M99 already has many flaws (expensive as hell, hardly reliable against anything but single, big targets,...), but it's so damn powerful that you actually need to keep it balanced that way. Otherwise, nobody would even pick the Railgun!
Finally, weight on its own has the purpose of regulating your "walking speed". Also part of the balancing! The gunslinger and SWAT have the benefit of being more nimble, totally in-tune with their roles. But one could choose to use the low weight of their guns to be fully stocked : if you want to have three SMGs, you can !
This being said, I do agree on two things you said :
>Ramping up the difficulty, not only with the amount of players but also due to their loadouts would be awesome. Difficulty-scaling has been a major problem of the game : it can go from very easy to ridiculously hard in a matter of minutes. I believe it would be a bit fairer to do it that way.
>Considering the amount of stuff every perk now has after 5 years in development, it's indeed a bit puzzling that there's really one meta-pick as far as loadouts are concerned... But I believe the best way would be to improve on the ignored weapons rather than allow you to pick whatever. Because in the end... You'd still pick the best guns.
Also, as Humam posted above, the issue is that it's not as "easy" as having light and heavy guns. If we look back at the Gunslinger and SWAT... They don't have any heavy weapons? So they could steamroll on the game with easy, using their best weapons? On the flipside, the SA80 is a mere T2 for the Commando, yet weigh 6kg... Already a medium gun? Also also, we got the upgrade system now... Could a light weapon become a heavy weapon?
Bottom-line would be that weight is a common-trope in video games : limitations. It's not only part of the balancing factor, but part of the fun too. Creativity does stem from limitation! Just like at some point you need to know if you choose to sell your weapon for a better one, but with low ammo... Or if you refill your armor but then you cannot buy a better gun... Or again, you choose to keep your T1 weapon for the whole game... You have to be wary of your weight in the latter waves. And I think it's part of the charm really... And part of the strategy.