I recently completed all the Bass goals and now I want to dive into Pro-Bass. I want use a realistic bass but as it stands HMX doesn't seem to have plans for Bass, so my question is Mustang, Squier Strat or Wait for a Bass.

Squier.
 
Don't expect there to ever be a Bass.

I would like to say the squier but it really depends on the model/make and if you can play it before hand. It's nice of HMX to allow this as an option, but if a new player is using the game as a gateway into the world of music, they won't really know what to look for in a guitar to see if it's properly set up. And buying guitars through the mail usually through off the intonation and the action.

Activision released a Pro Bass controller a month or so ago...
 

 

I'd say wait for somebody with a lot of tech experience to take the squire-strat and an actual bass-guitar, and make it into a pro-bass, but that'd probably be a while... and it'd probably be over $500 on ebay or something... and in limited quantities.
 
Another option is to do this yourself (if you know how)

I want a pro bass so bad. I pride myself on being a good bass player in real life and posting a score online would be awesome