First and most importantly, add an ability to switch to the classic mode for the browser plugins. Perhaps it is already possible, but I did not find out how.
Good idea. You can do it right now by adding one button on toolbar using the toolbars-customization feature:
Second, the ability to emulate or to switch to the native window frame look and not Win10 like one. I understand that you implemented the non-native frame to be able to put buttons on it. However, it is still possible to emulate the native window look as it is done in Firefox for example. Alternatively, I suppose that the people that want the classical interface does not care about these additional buttons. You could just use the native window with the classical interface. Do not forget that half of the windows users are still on Win 7 (and given that Exchange Editor is mostly used in corporate environment, this number should be much higher for your users). If people would want a Win10 window frames on their Win7, they would just install a corresponding Windows theme, they are plenty of them.
Firstly I want to say that isn't easy to support both UI-modes in one runtime, to be honest. As result - not many applications implemented both modes in the same time, as I know.
As you noticed, the RibbonUI requires to use the window's caption. In corresponding to nature of Ribbon. Also in ClassicUI mode is useful to take that place for some hot commands (it can be customized too). While it is useful in ClassicUI - yes, it changes the look of standard window caption/frame. In my opinion - its not so critical for big number of users while window's custom caption is looking accurate and working correctly. But we are looking forward for feedback..
Third, the easier to implement for you; to be able to switch to old icons. They had a big advantage of being smaller and thus save screen space.
In the near future we planning to add feature to customize icon size. With that you will be able to reduce it as you want. Tip: currently you may try to use new simple option to reduce large icons:
Thanks for suggestions.