This is kind of universal, and doesn't matter what game your guild play. What it comes down to is the size of the guild and how many that will show up, and how far some have to travel.

If you are local to each other, it goes easy to just arrange a meeting at someones house (the one most local to everyone, and of some size that got the space) and hang out.

If you got many members from all over the world it becomes a bit different. You would want to pick a time when people most prone to have some time off, like during summer vacation. You also need to pick a place that got room for everyone. If you lucky and someone in your guild live sort of central and got a huge yard/garden, you could like rent a huge tent and stuff.

Otherwise you would want to rent a place. Most schools out during summer and some allow to be rented and would be affordable. If money no object as we seen with some players in Stormfall, what you go with just limited to what you want to pay for it.

Also people need to eat and drink, if you like 50+ people it's not that easy to just pick a resturant or similar, then order for food to the place you rented more sensible. (and sort that out before the day itself :p). What level you go with is what you willing to pay for.

If people had to travel by far, they also sleeping arrangements. That could be as simple as bring a sleepingbag to the place you rented, or make sure there is affordable hotel, motel, inn, whatever close by. 

What people want to do at the actual gathering is quite individual. If people want to bring a bunch of computers and geek out or have a more traditional party/festivities. From my own experience the best one is when you wake up in the morning and got no idea where you are and the night before is a blur, but each to their own taste.

Oh and last, don't expect your league marshall (guild leader) whatever, will be stuck with the entire bill, because he/she is the leader. It doesn't even need to be the person organizing such an event. Lets face it, every guild got some people better than others at organize such. 

I been at a few in my own country over the past years, and a couple in Netherlands. As most of the guilds I been member of over time, been mostly Euro based, Netherlands is a central place in Europe to hook up and easy travel access. 

ALSO, if you want to make it a big one, why limit yourself to just your own league, you can make offers to leagues you play with and against too. Those people you demonize in game as villains and bad people, they are humans too, and can be quite nice.

ALSO 2: Shame on Plarium for not organize something :p