Honestly, I don't know if it's a failproof system but the only discord server I'm active in, is a server that feel like a chatroom first, I think that's the way to go if you want people to engage more.
We are small in it and don't promote the server at all, it's almost like a private server or a group convo' between friend.
There is no clear rule, in fact we mostly talk in one channel even when we share our work, links, tutorials etc so the conversation can flow naturally. We also allow controversial topics because we are human and adult, so we can "yell" at each other and still remains friends afterward but it's also because we want to grow and eventualy be educated in topics we are not familiar with.
The extra channels we created, were out of necessity like our project, drawing or gaming talk but it happened only because our conversation flooded too much the main channel, to the point where we felt like we only talk about one topic so the people who didn't relate couldn't feel "at home" anymore. (The server grew organically)
But even with the extra channel, we still allow ourselve to talk about our project, gaming etc in the main channel just for the sake of letting the conversation growing organically.
And fun fact, when I got in, I knew nobody, I barely knew the guy that invited me. But right from the start, the banter between them felt so casual, a place where people could be vulnerable that it made me feel comfortable to talk to them.
So naturally as the year went on, we started to support each other's works in social media. We elevate each other's work by providing interesting feedbacks/angles, I know the in-depth of their project and they know mine.
Of all the server I tried to be invested in, it's the only one where I'm active, I'm there everyday just for the chat to the point where it slowed down my productivity 
So I'd say, if we create a discord server I think it should be the priority to make people feel like they can talk freely without the feeling that they will be "punished" for saying something or posting something they shouldn't etc.