I think the World Icons sets have been really amazing and would love to see these continue. One thing I would like to see is one of the world's great cathedrals, such as Notre Dame or the Cologne Cathedral. It's been 50 years since LEGO has had a 'church' set (sets 309 and 1309 from 1957 and 1958), probably because the brand did not want to alienate any part of the market, but the World Icons series is the perfect place for these.
One difficulty about the World Icons line is that they are all very high priced items. I think it would also be great to add a second series of smaller sets. Something more along the line of the Architecture series created in collaboration with Adam Reed Tucker. I am sure that LEGO is familiar with the work of Arthur Gugick, who has made a great number of world icons at a smaller scale:http://www.brickshelf.com/cgi-bin/galle ... =torgugick
I could imagine a similar collaboration with someone like him.
Finally on the idea of World Icons, I've always felt that it would be great to have a series of small minifig sets based around world cultures wearing traditional garb. Different figs could come from different countries, like a Buckingham Palace guard for England etc. Think, for instance, of the animated kids you see on the Disney Small World ride. The popularity of the Collectible Minifig line suggests that such a line would be popular (indeed, the Mariachi from that line could be representing Mexico, and the mime's beret might be appropriate for France).
One line that I think is an obvious one is Greek/Roman myths. This would give great opportunities for heroes like Heracles and Theseus and monsters like the Minotaur. These are obviously stories that have survived and resonated with people for literally thousands of years, and, lest anyone think today's kids just won't connect, see the popularity of the 'Percy Jackson and the Olympians' books. Again looking at the Collectible Minifigures line, the Greek Hoplite is one of the ones that has many people excited.
Another line that I think would be great is fairy tales. This would have the advantage of appealing to girls, if my experience with my nieces and now my daughter are any guide, but would not be off-putting to boys as something appallingly pink. Another advantage to LEGO (similar to the Greek myths above) is that they can tap into well-known tales and characters without having to go into another licensing agreement, as these go back to the early 1800s for the Brothers Grimm, or, more appropriately for LEGO, Hans Christian Andersen. Speaking from the premiere LEGO Castle forum, I think both this and the Greek myths line would be popular among castle fans.
Finally, while I am generally opposed to excessive licenses, I did wish that LEGO had gone further with either the Spider-Man or Batman themes and extended the license to either Marvel or DC comics in general. There are so many great characters there to build sets around. As with Star Wars, these tap into a fan community far beyond LEGO that has been strong for decades. This is great as you get both adults and kids excited about the sets, as opposed to something like Ben10 that appeals to a specific age of kids but has little life beyond that. This could be done over multiple years, and sets could be grouped. So one year you have a series of sets all based around the X-Men, the next year a group of sets around the Fantastic Four, then the Avengers, etc. Or from DC, a year of Batman sets could be followed by a year of Superman sets, then a year on the Justice League etc.