Like it says on the tin, it would be really helpful if you got a notification on your phone when your watch was fully charged. And another thing related to that that would be extremely useful is a battery indicator on the watch for the phones battery, maybe right next to the watch's in the Quick settings panel.
Well you can check the battery status of your watch in the Galaxy wearable app. It's not a notification but quite easy to check. You can also get a watch face that includes the battery percentage of your phone. You just have to look for them.
Yeah, but they'd have to be something that you like. I found one that I liked that showed the phone battery but it was very inconsistent and rarely worked. I haven't found any I like since then. I like the ones with a ton of customizability and info on them. I am planning on making my own watch face using something other than the design studio thing, because that limits you a lot. But I'd like it to be a default feature.