GW2 is currently my favorite game and am really surprised to see it on HN frontpage.
I'm astounded at the amount of data ANet is making available. And, how cleanly and robustly they've done it - all in JSON.
Another bit you might want to play with is their TP (Trading Post) JSON data. These guys (http://www.gw2spidy.com, source is on github) do a really good job, but I'm sure there are tons more things, and ideas, that can come from all that data!
Good luck!
How do you think it compares to GW1? I have GW1, but found it a little lonely after leaving the cities because outside the cities, the regions were instanced.
GW2 does some great things with quests, their PQ-ish Dynamic Events work well, and their huge raid-boss-y world dudes are a ton of fun. The way you can join multiple guilds and the way you are scaled down in level based on where you are makes it really easy to play with friends regardless of your individual progression. It's definitely worth a look.
The open world is shared, and very collaborative.