I haven’t thought about promoting using .ui files to the general public, but I agree that it could work well.
To make Qt designer (with all custom ctk, qMRML, Slicer widgets) available without requiring users to install a compatible version of Qt, we would need to bundle Qt designer binaries in the installation package. It should be no problem, as it requires only a few extra files of a total size of 6 MB (designer.exe, Qt5Designer.dll, and Qt5DesignerComponents.dll; Qt designer plugins are already included). @jcfr what do you think?
In addition to bundling designer executable, we should add an “Edit .ui file” button to the Reload&Test section and add an ExtensionWizard template.