I would like to see charting capabilities within the tooltips.  For example, here is a creative solution for putting a bar chart in a tooltip.

http://kb.tableausoftware.com/articles/knowledgebase/barsintooltips

If there were a way to have a new canvas (sheet, dashboard and tooltip) allowing for any kind of chart in a tooltip.  It would be possible to have a map and assign a tooltip canvas with a bar chart, line chart, bullet graph, table or even a small set of charts.  A bar chart by category (apples, bananas) could have a tooltip with a line chart for sales over time for that category.

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As per input from Sean Boon....

"With Tableau 10.5, the Viz in Tooltip feature is now available"

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