So this is going to be a complex one so I understand if it gets rejected, but I think it's an interesting idea.
It's doable, even relatively easy, but it need to be well-defined and have a clear goal.
Pros:
-no more click and choosing fire mode every time, game speed will improve
That's arguable. In my opinion, a click takes almost no time. The most time is spent by moving the mouse to the hand icon and then back to the target unit. And this would still stay.
-improved immersion for the player
That's very subjective though. My immersion stems from action, not from UI.
-no more UI from the MSDOS squares era. (the rest of the game is slicker, may be the only place where the squares appear in game)
I love squares
-increase game speed with mouse (no need for keyboard hotkeys for quick fires)
I wouldn't say increase. The quickest way is still using keyboard, or a combination of keyboard and mouse.
However, if your primary motivation for this request is to NOT use keyboard at all, then it would be a (small) increase.
Cons:
-maybe need to use reserved time units since it's a different system... to see TUs for snap (on the?) left
Maybe some more details or a mockup?
Cons:
-needs to be an option so people can play the old ways
Of course. It would be an option, turned off by default.
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To recap:
1. The speed increase argument is not very convincing.
2. The argument for removing squares and something about immersion is what I as a player absolutely don't care about. But I understand that other players might have exactly opposite opinion.
3. The argument for not using keyboard sounds the most reasonable to me (even though I wouldn't use it myself, since using keyboard is quicker).
And the most important question: What is the primary motivation for your request? (so that we talk about that and don't get sidetracked)