how much money do you have to spend? i could drop $30,000 of your money based on the info you provided.
studio work is a very expansive proposition in terms of gear acquisition. off the top of my head....
4-5 studio strobes
various light modifiers (boxes, strips, umbrellas, grids) + flags, scrims, and bounces
various light stands + grips
laptop or desktop for shooting tethered
a 2nd camera
replacement for the 18-55mm
2 or 3 speedlights
specific lighting setups require certain gear. being as open ended as you were in your first post doesnt narrow it down enough to suggest anything specific.