Keys and guitar will be fairly flexible in terms of what they represent. I'm pretty sure, for example, the massive sax solo on That's How We Roll will be on guitar, but I think the sax solo on Race to the Bridge will wind up on keys.
To my knowledge, every track has piano, guitar, bass, and drums. That's the band's rhythm section. So most of our planning and experimenting is a matter of when do we jump from that, and to what. Putting anything on vocals does add the concerns for consistency- it's more confusing for vocals to jump from sax to trumpet to trombone than for the guitar to do so. We do have harmonies at our disposal, but... yea, there are enough concerns that we're not guaranteeing that we will do hornvox because we can't guarantee it'll work well or go over well. But hey, worth a try.
And these are the times we wished we had every instrument made as Rock Band instruments LOL.
Can you imagine having a full band with:
(and more of course)
The Cure 
My Chem.