New monitors would be a worth a thought. Despite the ones you allready have, a couple of reverence monitors could be a nice addition to your setup. (I currently use the M-Audio DSM1 Audiophile.....NICE!!!)

As for a vocal harmonizer and a fantastic music toy: the TC-Helicon VoiceLive 2. I'm still exploring all the crazy stuff you can do with that box. It also has a neat display and a set of cool knobs: when placed tilted on your desk it really looks fantastic

If you don't have one allready: a controller for your DAW would also be something for you. Something like the Mackie Control universal pro?

Want a cool software synth? Perhaps Steinbergs HALion symphonic orchestra is something for you?

Oh, and a neat electronic drumkit to create your own drumtracks.

Ehm.........I think I'm overreacting here. But I'm like a kid in a candystore when it comes to this subject