OK, let's clarify everything 
I haven't checked it on the Mac, I don't use the ARC USB on the MacOS at the moment.
Yes, "Always on top" is too much for me. I want to be able to cover the Totalmix window with Cubase/Reaper/Notepad/Video/whatever.
RME Support wrote:He wants something that temporarily brings the window to the foreground with the push of a button, but will let it move back again when the focus changes to another application. "Always on top" is always on top.
Yes, exactly!
When the Totalmix window is on the bottom (under other windows), the current behaviour while using the "Toggle Totalmix Windows" button on Windows is:
hide it,
show it - still under other windows.
And I suggest it should do:
bring it to the front
hide it,
show it AND bring it to the front.
I see the "1. bring it to the front" as optional but it is IMO the expected behaviour. The last "3. show it AND bring it to the front" is crucial. I think this should also make the TotalMix window active one (as at the moment the window shows somewhere but isn't the active window).
This might be important - It always shows on the top when I close the TotalMix with Alt+F4, but it doesn't show on the top when I close it with the toggle button. (Still it doesn't opens as the active window, so I need to Alt+Tab to it or use a mouse first!)
PS. I use the TotalMix FX 2.00 RC 1 x64 at the moment with the new design (beautiful and pretty responsive! I love it!)