An IS Expansion box w/ some missing Intro minis from the KS along with some Tanks/IS BA/Infantry stands would be solid.
Of the top of my head a couple companies of missing mechs to choose from.
Goliath, Charger, Imp
Jagermech, Ostroc, Ostsol, Bombardier
Kintaro, Dervish, Scorpion, Vindicator, Wyvern, Vulcan, Assassin, Hermes-II, Clint, Whitworth, Cicada
Javelin, Firestarter, Ostscout, Falcon, Hussar, Hermes, Thorn