Today I want to put Ill Omen on left click. I need to be casting it all the time, and being able to auto attack when it’s on cooldown would free up a whole hotkey in my build.
In general, I think all the skills should be enabled for left click. Let the players figure out what works and what doesn’t.
Something I would appreciate too, if not in GD1 maybe in GD2. So many times I’ve had build ideas that I abandon because I can’t have the skills I want on the LMB, only on the RMB. 
If you really want it and are willing to use some tools + modding - I think a behavior of having Ill Omen bound to LMB can be achieved.
first of all hard modding Ill Omen to work on a target instead of on the ground
then you could macro it to be spammed on LMB hold
I’m mentioning it because I don’t think 
has ever changed ability to bind a skill to LMB, at least for a few years I’m here. But you never know.