I think Nebula complements existing VSTis very, very well. There are already some fantastic ones with elegant workflow and outstanding sound.
For filtering, I highly recommend Cytomic's The Drop. It just sounds real and alive, and can be used in a lot of interesting ways with all the modulation options. It stacks well on the output of other VSTi also.
For synths, U-He's Diva and Bazille are very impressive, and sound even better with a bit of Nebula flavor on the outputs. Driven N-style preamps especially or things like the VultCult can add life and size and aggression.
Using something like NI Battery with Nebula AD/DA and preamps should be great as well.
For RJHollins, have you ever used Goldbaby's sample packs? He really has a knack for capturing analog and digital drum machines, processed though a lot of guitar pedals, filters, modular stuff, and plenty of tasteful analog kit. He has some which have been re-sampled through his Akai too.