The primary goal is building tools that other (typically less tech-savvy) people can use to create various types of content (often video game related) and to semi-automate repetitive tasks. An example here would be our texture selection / marking tool [1]. As a more advanced example, think of an editor found in most modern game engines, like Flax Engine [2].
Windows is the primary target for these tools, but I'd really like them to be also available on Linux to lessen our Windows dependency. I've used Qt in the past, before they introduced Qt Quick. I also heard about complicated licensing changes when they moved to Qt6, which made a lot of KDE devs worry. And stuff like not being able to download Qt without an account; or the framework coming with everything and the kitchen think nowadays, where I am only interested in desktop UI; no networking, no JavaScript-like scripting language, etc.
I don't want to build a complete UI system from the ground up, but there are certain points where I'd like to be able to customize things, like adding new widgets and having some way to render 2D things without needing a graphics API surface -- think HTML canvas. I feel like ImGui does a pretty good job here, giving you drawing primitives.
For state management I am mostly concerned with the life-time, ownership, and connections between objects. Where other languages, like C#, don't really have to worry about this due to garbage collection, in C++ you typically want things to be more strictly organized. I'd prefer a UI framework to facility object life-time management in a streamlined manner. Like, if it opts to use shared_ptr for everything, that's fine, but it also needs to prevent me from accidentally building cycles and provide a way to dump the dependence graph so I can see directly why a certain object is retained (and by whom).
To clarify the difference between C# and C++ here, think about how the implementation of an observer pattern is vastly more complicated in C++ to be safe as object life-time is not managed automatically for you. Copy & move semantics only adds to this in terms of complexity.
State management and user interaction are closely related here, as almost all user interaction results in state modification. Looking at HTML/JS frameworks, some leverage a 2-way data binding approach, where others bind data only 1-way and use events for the other way. In immediate mode GUIs I am updating the underlying state directly -- practically having the view and model tightly coupled. Here I'd like for a framework to be explicit about what is happening, without being too cumbersome to extend a UI with new functionality. E.g. I don't like signals that can be used across the whole code-base, where suddenly a function executes and you have no idea what originally triggered it. On the other hand, having to handle and forward every basic event from one component to its parent isn't an option either. If that makes any sense.
[1] https://github.com/ph3at/image_tool
[2] https://flaxengine.com/features/editor/