From what I recall, the Icon Heroes add on kit actually cost more than the Castle itself. Maybe that wouldn't be the case in MoTUC, since the Castle is multiple pieces and opens and such, but still, such a set would be huge, and I doubt Matty would ever sell it for under $250 (especially as, now that I think about it, Point Dread and the Talon Fighter would obviously have to have working interiors to stick our figures in).
Now, also as I recall, Toyguru reported that a significant percentage of people who pre-ordered CG never paid for it. I believe Yodafreakmaster and MOTU_Maniac reported similar instances of people arranging to purchase CGs through them, and then backing out once the time actually came. Add to that there were (and still are) a good number of people here on the forums who said outright they felt the Castle was much too expensive for what it was, and there are also a decent number of people who seem to feel it is just too big, and this add on set would make it much, much larger.
So, no, I don't think this would actually be very successful. I Point Dread/Talon Fighter only set might work (though I'm curious to see how successful the Battle Ram is first), but not all the other stuff. Too big, too expensive, and the interest in the line is dropping off rapidly. For the cost this set would be, I would much rather get a Snake Mountain, but that seems to be pretty much off the table at this point as well.