We have lots of properties on objects that change over time. For example, on a contact, we may have a numeric field type of "dollars_spent" to reflect how much they have spent. Currently there is no way to report against the history of a field over time (e.g. how did this dollars_spent field change over time).
It would be great to see a time series type of report become available against objects whereby the object field histories are able to drive the charts, much in the same way that lots of the analytics sections do time series reporting.