The sweep function may be a nice feature. I'd also include sweep time. The function generator I have will sweep from X to Y in Z seconds, but, one thing I dislike: it starts X to Y, and, when finishes, starts from X again. I'd like to see it go X to Y, Y to X, X to Y, etc...
As for "units", what I meant is: in order to enter amplitude, the units need to be in dBm (the image shows 15).
It may be a nice feature to have Vrms, Vp-p, etc...
This way instead of entering the value in dBm, the user can enter 1Vrms instead.
Also, I don't know if you will keep this open source, but maybe some users would be interested in purchasing one already built. Possibly consider selling them as a whole in the future.