I think part of the question (and problem) is, which part of QGIS do you focus on? The digitizing and geometry parts? The map-rendering part? Georeferencing? Vector and raster analysis?
Or to put it another way, what's the workflow that a simpler, more opinionated interface would solve, and what features could or would you sacrifice for it?
The most obvious ones. I'm still very new to GIS, but I find it amazing how difficult it is to simply draw some rectangles (of some specified dimensions) and lines on a map, and cleanly label the edges with distances. Annotating, moving, rotating things... is quite a pain compared to modern UIs. So I wonder if there's a slim and beginner-friendly open source version of QGIS.
"Most obvious" is still pretty subjective, though. It depends on how you wind up looking for a GIS tool.
- A lot of people come to it wanting to edit or annotate existing city/street maps with geometry, like you're describing.
- My first encounters with it were trying to plot points on a map to build a heat map, in a data analysis context, and I was coming from Google Earth/KML.
- The next time, I wanted to georeference raster maps to make more accurate vector data, and I was coming from Inkscape and Illustrator.
- Another popular use case is to work with topographical and height data, which is another workflow with mostly another interface in QGIS.
- And all of these can have very different output goals - slippy maps with raster or vector tiles, topo maps, 3D renderings and prints, GeoJSON and PostGIS data.
The "most obvious" app for each of those use cases would each be different, with different interfaces. And I can see where that would be good for beginners, or for limited use cases, but at some point each of those use cases intersects with another. Drawing bike lanes on a map quickly means working with topo data. Georeferencing a new base map eventually hits plotting POIs and drawing things like political borders as geometry. Outputting a heat map from data you plot in QGIS can work, but then you need to plug it into a database at some point so the data can be updated in a shared or automated context.
IMO what would be more useful for QGIS is a ground-up, no-assumptions review of its UI similar to what Blender did in 2.8/3.0. Find where they can consolidate redundant or conflicting tools and interfaces, redesign their mouse/keyboard interactions and tool palettes around how users intuit they should work, and provide alternate modes for the more complex workflows that existing users are accustomed to. Blender's redesign proved pretty well that an esoteric UI can be streamlined and modernized without sacrificing power user workflows.
But I don't know if the QGIS UI community is as large, organized, or focused on modern UI design as Blender's, which was a rather unique case in FLOSS. And it'd be difficult to divorce QGIS's GIS data-editing engine from its interface as a whole to build standalone tools with smaller UIs focused on tasks instead of toolboxes.
Plugins might be able to do this, which could be an interesting approach. Instead of adding features like plugins tend to do, one could ship a "flavor" of QGIS with pre-installed and -configured plugins that provide narrower, task-focused interfaces around existing features.
Or to put it another way, what's the workflow that a simpler, more opinionated interface would solve, and what features could or would you sacrifice for it?