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The biggest problem I believe we would face is 2 medics. A team would be able to get 2 ~6 seconds uber and push with 4 invulnerable combat classes. That would make pushing broken and 6s not fun.
Also, 2 or more demomen can shutdown an entire area with traps.
ComangliaOne of the things I'm most excited about and that really hasn't been mentioned, that with the new match making we'll probably see a bunch of either new maps or map improvements. Keep in mind cp_gorge, ctf_turbine, ctf_2fort, ctf_doublecross, cp_foundry and other maps that weren't really suited for competitive before might get completely reworked.
If you use CSGO as an example, Valve realizes people will only play de_ maps in competitive, so even though you are able to play cs_ maps in competitive matchmaking, they don't care if the maps are good for that mode. They don't even care to fix some de_ maps that aren't played in leagues (I'm looking at you, Aztec).
One thing I believe they could do is make viaduct pro the official one.