HDMI passthrough for sure, because my 4K-tv doesn’t provide eARC and I won’t purchase a new one for that. Also expecting rca inputs, like on the Sennheiser Ambeo Plus, to connect my hifi disk-player and make the soundbar my single audio system in the room.
I disagree with the changes you would like to see:
1) Full DTS. DTS is primarily only used in disc based movies. Discs are obsolete or soon will be. The original Arc was very forward thinking by eliminating it like Apple was when they eliminated disc drives in their computers. It also kept the cost down.
2) HDMI passthrough. Why? This is a sound bar not a receiver. Your TV should pass it's audio signal to the sound bar, not vice versa. If you don't have an eARC HDMI port on your TV, why are you shopping a $900 Dolby Atmos sound bar. That money should be spent on a better TV first.
3) Other sound inputs. Again, why? This is a digital sound bar for TV. Buy speakers if you want to listen to music. The music performance on this sound bar is already excellent and we, again, have moved past physical media. Why should Sonos go backwards?
4) Weightier more bassy sound. It's already best in class or top 2. As with every Sonos iteration, the bass always improves and I'm sure it will here too. But the current Arc is already excellent as a stand alone sound bar.
5) Improved stereo. Again, why? For being a sound bar and limited by it's physical shape and dimensions, it's already excellent. Buy separate speakers if you want true stereo separation.
Most of these suggestions would add cost to an already very expensive sound bar and they are completely unnecessary. It's easy to see why Sonos hasn't felt the need to update the Arc yet. It was way ahead of it's time and is still one of the best.