Total Commander

Nothing to explain here because the title say everything.
It would be nice, as in Chrome.
That will give a modern view of TC.

Thats a personal preference.
Rounded corners of tabs is just a cosmetic detail which doesn't enhance anything.
If it comes then with some sort of themes for TC including it.

I don't think there's any chance to make TC look modern (that's not a complaint). And isn't it still system control? The request should go to Microsoft, next time they redo the UI for no reason, they might for once try to improve it, instead of doing the opposite as they do lately.