What I wish for is support for connecting a 3.5" HDD.
Currently, I use an old ASUS WL-700ge at one place and a fit-pc2 with external USB enclosure at another, but the WL-700ge is terribly underpowered, and the external enclosure has severe limitations: the USB enclosures don't give full access to smart data, nor do they allow controlling things like power-management and spin-down. Furthermore, they are terribly power-hungry (it consumes more watts than my fit-pc2, even when the disk is not spinning!).

The esata port on the fit-pc3 is a step in the right direction, but I'd still like to see something like:
- a large fit-pc with a 3.5" internal slot (to make it useful to more people, it might be made to also serve as dual-2.5" slots).
- a fit-pc with a powered esata connector.
- some recommendation for a power-efficient external enclosure.

Thank you for you feedback. You can use self powered enclosure to avoid power issues. FitPC3 designed to be slim, so the 3.5 hardiness will strongly affect the FitPC3 dimensions

I did not mean to extend the fit-pc3 case specifically, since indeed the form-factor (and heat constraints) will require significant changes. I just hope to see a future "large fit-pc" or "fit-NAS" or "fat-pc" ... I'll let you choose the name.
Also the other two alternatives still seem perfectly compatible with the general fit-pc design: a powered-esata port, or a recommendation for a power-efficient external enclosure.

I liked the FatPC

Hrm, perhaps an eSATA/USB 3.0 HDD enclosure that the Fit-PC3/Intense can 'stack' on? (And/or a bracket that would mount them together?)

Cooling would be my only issue, as heat from the Fit-PC would likely work its way into the HDD enclosure.

I have been looking for a dual low power server setup for a while so I like this idea aswell.