I need a function generator and I like AD9851. What I really would like is a Function Generator Hat with programmable +/- voltage like the diagram below: 
This is not impossible, but the DDS, Amplifier and Programmable PSU on a small Hat is not realistic. Or – well – I need to think about that one. In the meanwhile I decided on a compromise as follows:
I can easily switch between 3.3V and 5V using SPC3819 and that alone is a start. AD8008 amplifies the signal at high bandwidth giving 100mA out which is decent and AD9851 deliver a very decent 70Mhz function generator. This should be very realistic to get in a Hat format.
I seriously would like to program +/- 10V or similar out, but I need to work on that. It is possible, but I have 2 challenges – (1) is the noise of a DC/DC so I need to dig into a programmable, linear PSU. (2) Amplifier at high frequency, but I wonder if I am ok with <1Mhz at this functional level. So, in short – the programmable amplifier can be a separate Hat.