justshipit
I don't know if this can be realized. You can provide a custom image (I will fix this to pass the flarum icon once beta 3 is out), you should probably take a look then.
The layout of the messages is pretty basic for now, so that will be updated in the future, too. But I like the stuff you posted there, I'll keep that in mind. :-)
EDIT: Just to answer your question: The messages need to be compatible with all connectors. The other connectors don't support that much fields, so I you will have to either wait for the next version (I'll see what I can realize) or to hack you way through in order to focus completely on Slack. :-)