I went with the Starcraft option, though there are a lot of other things that I could list that I would have liked to see made into a Legos.
Zelda is one of the main ones I would like to see done, and I've been suggesting that from the age of 10, so it actually seems like it would be a natural choice. Colorful locations (especially large dungeons and castles, which is what Legos seems to be made for), lot's of peripheral but distinctive characters, and just a lot of things that Legos seems to use when making sets.
Maybe I'm missing something here, but I really don't get why Discworld is such a popular option. Don't get me wrong, I love the books, more than a lot of people I know, but most of the reason I like it is the writing (dialogue, the way things are worded, and especially the footnotes). Though I do like some of the character, it just doesn't seem like the most visually distinctive world (compared to some other options), which means that Legos wouldn't have a lot to go with.