Mustang 90 has no trem and two P90's, right? From the guitars you mentioned a Jazzmaster or Strat would be a great addition in terms of sounds and things you can play with it. A Jaguar style could fit too, it has a siniliar scale length like a mustang. I have 6 electric guitars and wouldn't like to miss them, having some variety is fun! If you buy used you can always sell them if you don't like it without loosing much.

Also, I wouldn't give fender any money since the last lawsuit and their decision to sue the whole guitar industry. There are lot of other interesting guitar manufacturer out there!