I've done a bit of reading and checked out various Big Band compositions.
I know that these are the instruments that are very common in big bands that are not found in GPO:
Euphonium (Tenor Tuba)
Cornetto (Piccolo Cornet)
Saxes and woodwinds:
Clarinetto (Piccolo Clarinet)
Jazz Clarinet (tuned for playing in triad chords)
I'm sure there are many others not listed above.
Of course, guitars and drums. I'm not sure if we need these in Big Band since there are plenty of other libraries that offer great guitar and drum sounds for decent prices, but if they are thrown in for good measure it would be a plus for the convenience of having them in the same library.