We want to replace Qt with Avalonia UI for the editor UI, a native C# solution that should be easier to work with and faster.

That would be good to use Razor to make editor tools, to only have one langage to make everything. Like parsing Razor into Avalonia instead of using XAML or C# Avalonia API

IMO one of the best features of godot is that the editor UI is made with the same UI framework as your games UI. This makes creating editor tools in godot so fucking easy, no learning a new UI.

Following in godots foot steps, having razor be supported as an editor UI would be amazing. Sort of this, Avalonia alone would be better than Qt

Here is my opinion. I worked with WPF and Avalonia UI for a long time and I don't think it is suited for sbox.

Avalonia's main features are: design preview, cross platform support, data binding and css-like hierarchical styles. And even though binding is very good addition, but design preview, cross platform support not needed and advanced styling is very unnecessary.

I think editor UI should be consistent across all editor tools, must have a easy learning curve and good accessibility. Advanced styling would hurt that vision.
Qt on the other hand more strict in this regard. This would make sure that your own tools would still look and work similar to other people tools.

And I don't need XAML. It would be another thing to worry about. Another thing that would break development flow as you would need to constantly switch from XAML to csharp.

I don't see a way Avalonia would help with editor UI development and make it easier and faster. But you all are very welcome to change my mind. 

Allows opening up the base of the editor to open-source unlike what valve is doing, would be awesome for custom tools

Razor seems logical to me for writing editor tooling.
Although s&box already has a lot of QT niceties, such as the graph widgets and other cool widgets you can tinker with,

Every editor tool would immediately break if it relied upon editor widgets (which is about 95% of s&box editor tools).
I'm down for it if it's easy to port old QT code to avalonia for example.
But I'm concerned about the editor being broken on release for another 5 months because the avalonia rewrite branch was merged too early.

I’ve found Avalonia Declarative Markup to be very useful for creating Avalonia UI using pure C# instead of XAML.
It’s up to the team to decide what to use but just giving some light on it in case anyone finds it useful.

The original repo on GitHub is AvaloniaUI/Avalonia.Markup.Declarative. There are a few issues it with though, which is somewhat solved by some of its forks, including the-nexa-group/Avalonia.Markup.Declarative

I haven’t looked into Razor for Avalonia though.

Will it support languages from different countries?

Not done much with UI programming in S&box or in general, I did a bit of QT stuff a very long time ago, but nothing more than just experimenting. Regardless, here are my thoughts 

I would definitely say try and get Blazor going before avalonia UI. If it is technically feasible.

That way Blazor documentation and improvements can carry over, and changes/modifications made to it will improve not just games, but tools as well, and vice versa.

There is no redirecting Avalonia docs, XAML that needs to be learned or anything like that, you know how to build UIs in S&box, then you know how to build editor tooling for it.