There are maybe many missing measures in Google Analytics which could be useful, but the one I'm referring to is useful AND is available if you know where to look.
What is the missing measure?
The measure I'm referring to is Second Page Viewed on your site.
In this example, I'm looking to see the second page which within the [Visitors:New vs Returning] report. Filtering for New visitors shows me the most important path is visitors going back to the home page. (not set) is the bounces.
How will it help me?
Why would I be interested in that you're thinking - I can see my top entrance or landing pages, that's what really counts.
The main reason I find this useful is to do with understanding how site visitors use site-wide navigation.
Although you can see a forward path for a page from its navigation summary or Site overlay in Google Analytics (if you can get it working), it's difficult to get a feel for which are the popular navigation features across the whole site.
I recently used this measure to show a client that many visitors were clicking on "Contact Us" and then within that page, using the navigation summary, a store locator option. So we needed to make the store locator in a more prominent logical place.
Where do I find it?
This is the tricky part. It is available through the Google Analytics API, but not GA itself AFAIK. So to access it easily you need a clever free tool by Caleb Whitmore of Analytics Pros called Dimensionator which exposes a range of dimensions that can be ovelaid on existing reports in GA.
Thanks to Lucy Griffith for alerting me to the tool and Caleb for creating and sharing.
Dimensionator is a bookmarklet that you add to your browser and then select the dimensions you require when in the relevant report - secondary dimension is the main way I use it.
If you use Dimensionator already, please share how you use it. I asked Lucy for her most useful feature and she independently mentioned the second page viewed.