I think you're on the right path, and trust me I'm just a little upstart with a few recipes under her belt (you can take my suggestions with a big tablespoon of bacon salt)
I absorbed "Brew Like a Monk" and what stood out in my mind most was the simplicity of the grain bill - I really think that Belgian dubbels and tripels are all about the yeast..
but then you have Unibroue ...
I am actually brewing today with some washed 3864 from a blond I brewed in the spring. That yeast is a monster - and a pretty cool one as well (the starter is rockin like Dokken).
Anyway, as I understand it, Belgian Caravienne is a caramel malt like Vienna malt - lighter in color than Munich malt. I brewed a partial mash dubbel back in August where I used Carabelg (Caravienne) and a tiny amount of Aromatic and Caramel Pils (because I think I read somewhere that Caramel Pils added to the mouthfeel of the finished product). I'm still not sure it's going to make any difference (the beer is still too green to realy tell)
I was thinking you were looking for malty sweetness from the Munich malt, and thinking that the Caravienne wasn't going to add much new conversation to the party. Since the 3864 doesn't thwack you upside the head with plum/raisin/dark fruit - it's up to you. The yeast gives me an apple/lemon/floral profile.
As far as the Coriander goes, it throws lemon, and can overpower really quickly if you use too much. (i made that mistake). I think it works better in a pale /blond/wheat ale because the flavors seem brighter and less earthy/warming. What about cinnamon, peppercorns, or a miniscule amount of star anise (licorice). Warming spice.
Oh yeah... it was really easy for me to overshoot my OG .. and although it finished at 1.01 it was one potent beast when I put it in the bottles. I don't think I'll be cracking another bottle until after Thanksgiving.