In my opinion the spells which certain heroes cast should burn into the floor of the maps. Tyrael should leave yellow trails of his spells on the floor.
Diablo should burn the ground when he fire breathes.
There should be a map with tiles and they break when Sonya uses Siesmic slam. The bushes should get cut when she whirlwinds too.
Leoric’s tomb should be destructible.
Zagara’s creep tumor shouldn’t have a timer.
Pieces of mech should last forever on the battlefield when it explodes.
I wish I could implement all this stuff but I can’t, I live in the UK. 
Bushes should keep growing and multiplying.