but in open space
I hate to burst your bubble but it's not a problem even in "open space". If you can't do it don't you dare brake it for those who can just because you are too lazy to practice!
Once again - do not touch that mechanic! Since there is no "universal" solution for that and fawing one direction will completely screw up another one.
And the point of AoE cast zone because it's AoE! Next wine will be about, i forsee, "Why Miasma II explode around me and not arount targeted mob?! Fix that ZOMG!!!!1111oneone"