This is my exact line of thought, also.
-20% Physical Damage Taken would be awesome. Probably too awesome for the Development Bros to consider putting it on the Rainbow Stick, but I'd be overjoyed if I am wrong.
I'd prefer +9~ STR/DEX/INT/MND/ETC. to most things because base statistics do more for my Charged Whiskers and Everyone's Grudges than whatever else might end up on the Rainbow Stick, even if it's not a huge bonus. The everything burrito nature of the bonus would actually work really well for the weird modifiers most better magical Blue Magic possess.
I wouldn't mind having +15 Ranged Attack for Corsair, but that's about the only secondary bonus I can see "non-mage stick people" making use of. Cure Potency will be a nice space-saver for Blue Mage if it's included, though.
Also, I want to echo a hope that many have expressed. A hope against hope. A hope that shines in the darkness like the wrapper the last uneaten Cadbury caramel egg on May the third. The hope that some kind of combined version of some group or groups of Magian staves is coming.