Spring reverb is always nice in a hardware effects setup. Doepfer has a nice simple one.
You could also just get a Finegear Dust Collector to go with the rack, but that might be a little overboard.
Another option is a Poly Hector. It has Rings and Clouds in it plus a lot of basic effects, modulation, and routing tools that would take a lot of space in another rack. It's a touch screen, but it has a very effective interface that makes it easy to tweak and adjust.