1. No, I actually don't buy these sets though might in the future.
2. Mostly for the appeal, though if it is a cheap set and will compliment one that I have or offers a lot of minifigs I will be tempted to get it.
3. The working Big Ben idea that was posted previously would be *amazing* and would definitely be a great showpiece.
4. A huge castle. 15+ minifigs. It would be huge, it would be detailed, it would take a solid month to build. It would be awesome.
5. I would love to see a greek or even roman theme, heck you could throw in mythology elements even for some opposition. Ever picture a LEGO trojan horse? Hoplite minifigure? How about a kraken? Zeus minifigure or Apollo? SO MANY POSSIBILITIES!