Depends on your budget and weather you want/ need hard luggage. I think a topcase for passenger is almost necessary. Check the above mentioned hepco & becker sitefor a rack and topcase.
Scroll down to see the rear rack options.
Scrambler
Hard cases here:
Hard Luggage
With H&B, it's a matter of matching topcase and rack, they don't all work together.
Also, GIVI has hard side cases that are 30liters each, but expensive for the whole set, but no rear rack.
GIVI My motorcycle – All the accessories for your motorcycle
Ducati sells a rear rack:
SCRAMBLER, ALL, Ducati Scrambler Rear Luggage Rack
as does AltRider
Luggage Rack for the Ducati Scrambler AltRider
If you want soft luggage with no straps necessary across the passenger:
Ducati Parts, Accessories & Apparel
I have the Scrambler Classic side bags. They look good, but are very small and expensive. You also need to buy the racks that go with them.
The Urban Enduro Bags are the same size but waterproof.
Finally, SW-Motech legend Gear. If you choose the The LC1 and LC2 do not require straps but do require the mounting racks.
by products - Legend Gear - SW-MOTECH