Couple of suggestions pertaining to expanding/collapsing nodes:
There should be an expand all/collapse all function.
There should be a way to expand all nodes to a specific level. This could be implemented as a keyboard shortcut, by typing the level number you want to view. For example, with no nodes selected, if you type the number 0, that would be the equivalent of the collapse all function, and only the root node would be displayed. If you type the number 1, that would display the root node and its children. If you type 2, that would display the root node, its children, and their children, etc, etc, etc… Since there can be multiple mind maps on a page, this would affect all mind maps. Additionally, you could implement this at the node level. For instance, if you have a root node selected and you type the number 1, that would display the root node and its children, for that mind map only. If you have one of the root children selected and you type the number 1, that would display the children of that node only.