From the window decorations, I actualy like Midna Flat Wide Grey and I even expected it to be the default selection, but for some reason, it isnt... either that, or blue variation. I wouldnt go for default Breeze, cause its almost on any KDE distro as default. And with how different the Midna theme is from others, it also needs to be different with window decorations.
As for the wallpaper, again... there is that white one with lines and KaOS logo. Beautiful wallpaper indeed, but why its not default selection? Instead, there are these liho flowers... its also pretty nice and interesting wallpaper, but doesnt look like one to be the default one to me... for login/lock screen, sure, but for desktop, there should have been the one with KaOS logo. But you want to change wallpapers anyway, so im just mentioning how I feel about current setup.
Also, one little detail... the main panel on the right side. I tried to use it like that, and I got used to it actualy... now I really dig it, but any reason why did you decided to put it on the right side? Or was it just for the sake of differentiation?
There was still one small thing that bothered me after first boot up though... Why is that panel so small? Its so small, that it doesnt even make sense and doesnt look good like that. But if you increase the panel size to 50, it makes much more sense. Everything is bigger, time and date is readable, and it gives much better first impression imo. But I guess it also depends on dpi and screen resolution, huh?
Anyway, I have been watching some videos on KaOS, and first impression from people is usualy bad or average... not because of how it looks overall, midna theme is just sleek, but because of that panel on the right. I dont suggest you to put it to bottom, but maybe... how small it is is one of the reasons why they complain.
My current setup looks like this: https://i.postimg.cc/J740ygXf/Screenshot-20210128-202232.png
And I think it does look more "default" than current default one. Also, Tauon Music Box with white theme really does fit well into Midna theme. I dont wanna make advertisements or anything like that, but for the look and feel... I think it could be nice default music player for KaOS.