One thing that's bothered me about OBS is that the window usually has a lot of dead space in it. Most monitors in the past 10 years have been widescreen, so the current OBS window seems like it wastes quite a bit of space. I would suggest putting the scenes/sources/buttons/bars in a vertical list stacked on one side of the preview window. Ideally, the layout would be responsive or fluid, but I believe by default it would be better to have it on the side (my weak preference would be the right side, but it could also be the left.)
Rough MS Paint mockup attached.
Rough MS Paint mockup attached.