Somewhat intriguing proposal, but I think it's a gamble. I don't have a lot of confidence that any player coming to the Jazz in this trade will ever be better than Millsap -- or even Harris. I'm not sure I completely buy the idea that Millsap is gone after next year regardless.
If #8 is Marshall, why do we think he'll ever be better than Calderon? A little better passing, perhaps, but just as bad (or worse) defense and clearly worse shooting/ability to create own shot. I wouldn't predict it, but it wouldn't surprise me to see Toronto take a pretty substantial jump next year if this trade went through. I'm not sure a late lottery 2013/early non-lottery gives us much (which is where we could be as well with our own pick without Millsap/Harris; and the GS pick). The safest prediction for picks in that range in that draft is mediocrity.
I'd probably be fine if something like this happened, but I wouldn't be overly excited. I think it's a gamble that could pay off, but the strong odds are that it's a fairly lateral move.