How do you give a Prophet 5 an arpeggiator? Get a Pro 3!
My Pro 3 and Prophet 5 are in different rooms right now, but are linked together via Bluetooth using WIDI Jacks. I set both synths to the same MIDI channel and press the Arpeggiator button on my Pro 3. Then on the Prophet 5, I set Local to "SOn" to allow panel controls to work. Now MIDI notes are flung through the air from Prophet 5 to Pro 3, where the Pro 3 plays an arpeggio and flings it back to the Prophet 5. It's a lot of fun, and takes much of the opportunity cost away from going with P5 over Trigon 6.
The temporary downside is, with the Pro 3 in my office, I don't have immediate access to tempo, arpeggiator mode, etc. Fortunately, I just ordered a two-tier stand, and my instruments will soon be united. I'm still going to have all my connections via Bluetooth, even when the synths are in close proximity. There's no latency that I notice, and I can seamlessly bring in my computer or iPad or whatever.
Of course, this technique also works with the sequencer. I've had a little taste of sequencing Prophet 5 parameters by assigning MIDI1 CCs to sequencer tracks, but I'll need to dig in a bit more on this to make it really useful.
I had originally considered selling off my Pro 3 to get a Hydrasynth, but I don't think I'm going to do that. I have tons of time invested in the Pro 3 (and I'm still learning!), and the two synths make a lovely pair.