Another bass player here--very glad to see the option!
The PS3/X360 instrument training game "Bandfuse" (which had the misfortune of coming out around the same time as RockSmith and went bankrupt within like a year of release) had a really great circle-of-fifths training mode...but only for guitar. :(
Yeah, its just that I have to pareto-prioritize. The more time I spend on more niche market features, it takes away time from features that 80% of the people want. I'd rather come back around once I have a fully functional product for guitar players and then start serving the custom tunings, the alternate instruments and so on..
I would like to finish all the modules for the guitar before I venture into other instruments. The bass so far has been kindof easy..just hide a couple strings :)
My one suggestion there, then, is to perhaps make the guitar/bass toggle more prominent to grab the attention of four-string aficionados.