The LMGs in 4.X should act like actual LMGs. They should do little to no damage against buildables. They should have high ROF, lots of recoil and ability to deploy a bipod to reduce the recoil by a lot.
Unturned needs real bipod deployment like arma. Bipods would work like you expect, it would be great for snipers and LMGs.
Since nelson added more advanced attachments and ability to change barrels, I think you should only be able to put bipods on LMG/Marksman barrels, that way you could turn an AK47 to an RPK or an an ar15 to a marksman rifle.
You should also br able to change recievers to go from a 5.56mm to 7.62mm round etc.
Melee weapons in unturned should have more depth, I think you should be able to have a blunt and stab attack. Stab would cause your opponent to bleed and blunt would do more damage and stun zombies. I also think you should be able to add certain things to a melee weapon, like a barbedwire around a baseball bat, jagged edges on knives etc.
Snipers in unturned should be a one shot kill to the head no matter the helmet.
Snipers should have very high recoil, very high damage, limted clip size and clunky to move around with, making it hard to do cqb with a sniper.
SMGs should have high ROF low damage and easy to move around with, making it excellent for cqb.
Pistols should have medium damage, very very easy to move around with and have a fast draw time.
Assault rifles should have medium damage, medium to high ROF and be the middleground of weapons.
Now for weapon degradation… I think in unturned 3.0 its supid,unrealistic and just annoying, in 4.x it should take much longer to degrade a weapon and you would need certain tools to maintain it, gun cleaning kits etc. Also the degradation should not affect damage, only accuracy and looks. A barrel may break and you are forced to find a new one to replace it.