Perhaps this is a bit out of scope for web analytics, but it would be really useful to be able to ingest and report on data where I am able to define the schema of the facts being recorded. For example, in the case of internet video, knowing when someone clicked on an interstitial ad (both in the context of the playlist that they’re watching as a whole, and the time-elapsed of the ad) would be rather useful and interesting from a data-mining point of view. There are many other examples and use-cases I could think of for this general feature.

So; the ability to be able to define my own stats facts using piwik, then the ability to ping into the facts table from my web application, essentially. Something like fact-id, fact-payload, where fact-payload is a JSON or form-encoded packet of data (probably IDs for separate reconciliation against transactional data), where on receipt of the ping piwik will route the fact into the appropriate backing store with due alacrity and verve.
Keywords: toogeneral