Two data structures that I think are important are trees and graphs, to store hierarchies and interrelationships respectively. I know these can be implemented by database tools and with scripting, but often these are a little out of reach of the average user. What the user needs is a highly visual way to store data in a tree or graph format: to be about to convert array data into tree/graph data; to be able to paste in; to be able to drag and drop. They need to be able to define additional properties for each node, and to make properties of one node, and relationships between nodes formula dependent. And they also need to be able to export the data in a visual format, e.g. for putting in a presentation.
The challenge: make all useful data structures accessible (and hence visualisable) to the average computer user, and expand their minds!