I think most of these ideas are great but actually too consequential.
Chargers should have an LED that displays red while charging and green when finished. An amber LED toward the end is acceptable. I'm not making this up. I have an item that only displays solid red, when it's finished charging.
I'm partial to blinking for charging and solid for charged. Allows single color LED which is cheaper. Then if you do use multi-color or RGB you can turn red for incompatible/error.