Well,there's now a whole other page so I'll just put my two cents there about LMGs:
1)It certainly depends on the LMG about accuracy. Of course most modern LMGs are made to be as usable and effective as possible , so I'm guessing they're lighters and more accurate. But I'm not sure than old machine guns are that reliable and could then be an effective way of adding an LMG (I'm thinking about the M60 that could easily overheat for example ,could be an interesting mechanic , or the MG42 which was heavy as hell). Plus, I'm sure that most LMG aren't that accurate if you "fire at will" . My best bet is that it would be better to fire short bursts and/or use a bipod in order to do so.
2)KF2 is a highly mobile game? Then it's a good way to turn it upside down! I don't know about you ,but I love when a game allow me to experiment different playstyles , and if I can for once defend a small zone without worrying about a gorefast , I think it's worth a try! That's something that bothers me a bit in KF2: the only viable strategy is to kite everywhere... I miss the different opportunities the first game offered.
3)I understand that the mag size is bothersome ,as it is very important ,but then maybe we could only use LMGs with a reasonable magazine? Nothing more than 60 bullets for example.
So in the end,here's how I see the LMGs being done (if they are ever added in the game)
+Large magazines
+Powerful
+High rate of fire
-Takes a long time to reload
-Weight a lot (so you're slowed down badly and can't pack another gun,besides a pistol)
-Can overheat
-Expensive
-Not very accurate if you spray and pray
So we would get a powerful weapon category that cranks up the commando(or eventually the SWAT?) in both his advantages and weaknesses