KDE3 enabled at least two fullscreen background imagery types simultaneously: colour/gradient, image/slideshow on top (one could choose either or both or defaults, but images/slideshows not fitting screen are surrounded by colour/gradient behind.) P5 still doesn't have gradients or slideshows, but I'm not requesting one more/first, but overall option for simultaneous fullscreen image/slideshow over fullscreen colour/gradient (each pair in one option, so you can have one from each, i.e., all choices of nothing over colour, nothing over gradient, image over default, slideshow over default, image over colour, image over gradient, slideshow over colour, slideshow over gradient.) Maybe there were many other great details I don't recall, like not just image/slideshow over colour/gradient, but images could be translucent... one would have to check KDE3 (GUI with best appearance ever.) It seems P5 has all single-image settings unless one hasn't changed: one I don't see now, that I used for slideshow, was 'centred maxpect,' whatever that means (centred images, maximized to two closest screen edges.)
P5 does have slideshows. I'm not adding the option of choosing what you have underneath an image. It'll clutter the UI for a very very niche feature. Gradient as a background we could add. I don't understand what this extra aspect ratio you want is. It sounds like Scaled, Keep Proportion.
Gradient would still be nice (currently I just make six versions (light, medium, dark, and duplicates 'redshifted') of all gradients for five or six or seven monitor resolutions (plus pads/tab(let)s/phones.) Thanks for fixing slideshow (and adding at least one colour behind can be changed; ) the new aspect ratio description seems equivalent.
update: checked on more, below. Linux/KDE Plasma: KDE Neon User 5.6 to 5.25.2 (up-to-date,) Slackware64 14.2 stable to 15.1-current (up-to-date) KDE Plasma Version: later 4 to 5.25.2 KDE Frameworks Version: later 4 to 5.95.0 Qt Version: later 4 to 5.15.5