A couple of Electro-Harmonix things I've had a jones for for a while are their HOG2 and the Ring Thing. The HOG2 is capable of doing some elaborate harmonic resynthesis/modulation, especially in tandem with external modulation sources. Drawback: it ain't cheap. But even with the cost, I'd suggest looking at some demo videos in which the more complex capabilities are being shown off.
The Ring Thing, OTOH, is cheaper...and WAAAAAY cheaper than the thing it can emulate: a frequency shifter. Look a bit more at this box and you'll note that the panel has indications for modes that only do up-shift and down-shift. This is its 'sideband modulation' mode and such, and this gives you the ability to access the up and down frequency shifting on individual outputs. If you've ever looked at what the few freestanding frequency shifters cost (and survived the sticker shock!), I don't have to tell you that this box costs NOWHERE near what the Bode-type units do. To be honest, if there ever was a reason for Eurorack users to add an effect send/return module, this might be near the top of the list...and if we're just talking pedalboard alone, the ability to create the strange, alien, trippy frequency-shifted spectra for this thing's outlay is a no-brainer. Calling it the 'Ring' Thing is sort of a misnomer, really, although it will do those things, too...the real draw is the shifting.