A bazooka, because trying to launch a full size bomber off a carrier requires more than just a catapult.

I wonder if Hornet was a ship girl in the Kancolle game, would she get a special plane loadout? It would be bloody cool to see her launch B-25s

If there will be carrier-launched B-25, it will likely be exclusive upgrade equipment and they can still be equipped in other carriers with penalty attached (like Yamato's 46cm)