An output module might be a nice touch as well as a MIDI to CV converter if you plan on playing in sync with a DAW and recording.
I would ditch the Plasma Drive. Not because it's a bad module... it's a great module. But it's really expensive and quite an HP hog for a small case. Leave that space open so you at least have some wiggle room for additional, smaller modules.
Don't be in too big of a hurry to fill your case immediately.