"...These aren't problems. You can create historical and fictional maps..."
The problem it's that if you give science fiction units to mapers in WW2 games... a majority of this mapers use this science fiction units in the majority of maps MP or SP... and the realism disappear practically completely... I dont know why, but a lot players like the very, very, big prototyps tanks ? (same if real big tanks KT, Eleph, JT, ST... exist already and well used in WW2... it's a no-sens).
I know well HS serie, and it's for that I have totaly stopped play in MP with it... and same very rarely in SP also. If I want play science fiction games, it exist a lot to science fiction games very better... And it's exactly similar if you leave (laisser-faire) the choice to any nations in MP game... and same in HS, you see Germans + USA/UK + USSR in same team... and if existing "Aztec" nation with tanks in Sudden or HS, you will see they in ground also

...
"...The prototypes are very funny..."
Not for me... For me the most funny = the realism feeling & immersion + fun = the best interesting (the "chess game" are funny or interesting ? the 2 surely... but not only funny). The prototypes must stay in games or mods specific for that (type named "secret weapons" or "if WW2 during more long time" by example).
So "very funy" in "secret weapons" science fiction mod maybe... and HS is surely become that. It's the big problem of commercial games (same SS2, HS is commercial in start), want to be attractive for the greatest number and so for the more young (the largest players of video games).
And understand well me, I have nothing against science fiction or "secret weapons" mods and games, if at side exist serious games or mods also. But if you leave (laisser-faire) the majority of players (young enough for the most part), the serious games and mods +/- historical will disappear...
The problem it's also that a lot mapers dont know history. By example in +/- historical mod same RWM, a lot mapers think that "Black Prince" and "US heavy tanks" are were used in WW2... while never. Or identical for 1940 french tanks used in scenario map by French in 1942/1945 "Free France" while never also and you see they in 1945 in Germany... very good science fiction immersion, it's sure. Very funny ? not sure. If you dont supervise a fiew, it's the big shambles. They dont necessarily deliberately but by lack of knowledge.
Besides, that's exactly what happens with the Pershing... only 200 used in WW2 only from january1945 (and only start progressively to arrive at this date, not all in once... maybe only 50 or 100 in april 1945 in ground ? Besides only 2 destroy in WW2

) and you have already see a MP (or SP) RWM (and I speak same dont about HS2...) map with USA nation without Pershing ? me never.
It's my point of view