I got the battery replaced on my Series 5 recently, as it required a charge twice a day.
Battery charge duration is the update I keep hoping for. Yes, I know the Ultra battery lasts a couple days. That's still not long enough.
While I was waiting for my Series 5 to come from Apple (they actually replaced the watch with a new Series 5), I wore one of my old fashioned solar powered watches. The batteries on those doesn't run down if the watch is worn and sees some light during the day.
Obviously, the Apple Watch has far more functionality, and I prefer wearing it, but boy I'd like the battery charge to last longer. My Apple Watch has let me down when traveling, and in other cases where I can't charge it. Low power mode helps, as does turning off the always on display.
Sigh