I would love to be able to Tag nodes with image markers.. things like a TICK or a CROSS are great for testing. Other Tags could include progress icons and status icons

This would be a key element in adding some means to sort children of a particular node. will vote :)

I would like to find and list all nodes with a specific tag. I'm assuming this would be part of such a feature. For example, particular nodes may require discussion with one or more other people. So I can tag all items that require discussion with Alice with the tags "Discuss" and "Alice", then list all nodes that have both these tags.

like in hackmd.io

Yes, this is a must. Often ideas have different layers. I structure mine by domain and would like to add tags about relevant project, places etc.

This suggestion likely subsumes my own. Vote!