What ROM could be used on an HTC Desire bravo with non-functioning physical buttons
You can install the Xposed framework on a rooted device. 

There are numerous modules that will enable softkeys on the device. Have a search in Xposed in the "Downloads" section for "softkeys". 

One example is here and it has the Android L style soft keys. In case the article goes down here's the XDA thread with the apk