there are a lot of backing tracks available online.
you always free jam to that...
the idea of a scale is that technically any note fits the harmony
(i.e. key you are playing in).
in music any note fits really, its just convention (and some math)
that has established what people find harmonically pleasing.
the only thing that i would see as an enhancement would be for the player to choose the key,
and yousician mesures if the notes you play are in the scale you are practicing.
to improve proficiency you should be able to set the tempo.

Would be good to have a few basic looping backing tracks (drums, simple bass) for noodling away to so we can still log as practice time.

Just some drum and bassguitar loops, repeating 2 (or max. 4) chords for free playing. Like a light mode of backing tracks. In different styles.

I was thinking of this too. Maybe there could be genres to play along with, and the app would maybe give the key changes, and the player would move the scale to play along with it.

I don't know how they would score it, but it could be a great idea for freeplay, outside of the gamification.

Interesting idea!

Yes but as part of a combined practice room with lots of more specific challenges. Speed trials, scales, continuous play lots and lots of challenges that are for skill advavment, warm-ups, etc.

I'd like to see a mode where you basically need to pick a single note only, but you could select the pace just like with metronome. I'd like to make some stamina/rythm exercises simply by selecting a note and picking it up and down for few minutes in a row in constant speed, and see the feedback on timing.
There's couple other things too like this, basically what you see here:
http://www.guitarworld.com/node/18629