As I said, kappatch is great but there are a lot of drastic changes which Relic might not even consider. I'm talking a mod, which is the core game with the some suggested but limited changes
Here is an example, everyone is fighting over PTRS and Cons spam. There are a lot of suggestions which can be tested onw by one and a lot of arguments may settle.
But your point is valid, player contribution is the key.
define a "drastic" change. is removing sprint from snipers a drastic change? is swapping the su-76 and t-70 a drastic change? is roughly doubling the katy's effectiveness a drastic change? relic makes all kind of stupid and unwanted changes and many of them are drastic. the problem is that they always fuck something up, usually badly.
basically, i don't trust relic, i think drastic changes are needed for the general health of the game, and i don't have a problem with them as long as they're good. idk if you've played elite mod for dow2 but it made HUGE changes to ret while the game still plays the same and feels the same. it's a lot of fun.
as for testing changes, that's pretty easy. ptrs cons for example. make a scenario map for a beta where one player is always soviets with the tank hunter doctrine. i'm sure they could get people to play it if they offered a decal or achievement or something and stuck it in the game as a challenge. it would matter a whole lot if it was balanced or not as it wouldn't affect rank and it should generate a lot of data if it was handled right. it would require significant work on relic's part (choosing a map and setting everything up) but i think the pay off would be well worth it. part of the problem with mods to test stuff is they don't get much traction.