Although I'm quite happy with my X-T3, I did have the X-S10 for a while and appreciated its grip and small size. As I shoot mostly jpegs and enjoy creating my own film sims, the X-T3 is the superior camera; it can store custom film sims without needing to also store other camera settings, which was required on the X-S10, and (I assume) on the X-S20. I think the custom banks are limited this way on the X-H2/2S as well. I don't see why Fuji can't allow the user to keep custom film sim settings separate from other global camera settings on these cameras. I do like Classic Neg though and miss it from the X-S10.
The other feature that I found useful on the X-S10, especially for timelapses, was the in-camera exposure smoothing during interval timer shooting (useful for day-to-night and night-to-day "holy grail" timelapses), which is not on the X-T3 and similar-gen cameras (although the X-T30ii has it). Fuji has been very slow to make any changes to their interval timer functions, adding exposure smoothing recently, and then adding Interval Priority on the X-T5. It would be nice if they could also add 4-digits to their number of shots taken with the interval timer (currently at 999, then infinity, which means the timer has to be manually stopped if more than 999 shots are desired). Both Nikon and Panasonic allow 4-digit limits, so there's really no reason not to. If the X-S20 has the latest interval features and film sims, I may be interested in it. Of course I also want in-camera timelapse video rendering, star trails, and starlight AF features, but Fuji can't seem to figure out how to do that with their X-trans sensor cameras... ;)