In the specific case of return followed by break, I wouldn't bother. The possible benefit is quite small but like any code changes there's the chance of introducing bugs.
However, if you want to take a crack at making some changes, I draw your attention to libqb.cpp and the functions contained therein. Many of them declare random temporary variables as static, which has the same effect as in BASIC - the value is preserved between calls to that function. However, the vast majority of these don't need to be static at all.
Having all these static variables means that a) none of these functions are able to be called recursively, b) they are not thread safe (qb64 uses a few threads of execution) and c) waste memory, because all static variables in a program must be allocated all the time (non-static variables are just allocated on the stack when the function is called them thrown away.
So, one thing you might consider taking a crack at is going through all these statics and removing them. If you do decide to take this on you're free to do as much or as little work as you like. Basically it would involve looking at each case and ensuring the static behaviour isn't being used, then just removing the keyword. If a variable is always assigned to in a function before it's ever read, that's a guarantee it doesn't need to be static. If it's read before it's set, that's a sure sign it does need to be static (or it's a bug!).
As a nice side effect, this'd take you on a nice tour through the source.
