One thing that I don't think gets enough thought or credit is the thought that people will actually be using interfaces. That sounds like a "duh" thing, but really think about it for a second.
Especially in communication interfaces, people will be receiving life changing information through interfaces. For example, you might find out that you're being cheated on, or that a loved one died. And an interface shouldn't be adding insult to injury during moments like this.
Imagine if you found out that a loved one died, and then a ghost flew out of the message. Or if you broke up with your girlfriend, and an animated broken heart appeared on your screen.
This isn't to say that you can't have some fun when designing interfaces. Interfaces for less serious things can usually include more playful elements, for example sticker suggestions or fun animations. Same thing with giving people options. People are definitely breaking up over Discord, however nobody has sticker suggestions enabled, so I don't think there is any harm.