There’s a lot of discussion scattered over various threads about open world and closed worlds.
Open world - you can play any character from anywhere, and it will be worked into the larp somehow.
Closed world - the setting is fairly closely defined, and you have to create characters that fit it.
There are advantages to each.
Open worlds (like the German larps, Lateral Worlds, and to some extent the Gathering) let players be anything. You can have characters from novels, anime, and battle games like Warhammer in a grand crossover alongside original characters. This gives ultimate flexibility to the players and lets them take ongoing characters between any open settings, and use the same gear and background.
The advantage of closed worlds is fiction emulation and closer integration. Most fiction takes part in closed worlds. Multi-dimensional fiction that crosses over with everything is rare (even the Multiverse setting that Mike wants to use has a specific set of dimensional cross-overs, you couldn’t play Warhammer orcs in it). So if you want to play a larp that feels like some fiction you like, most of the time a closed world will get closer. Closed worlds also promote integration between characters, locations, items, and events. In a closed world it’s more likely that you’ll have things in common with other PCs, and having things in common (even if it’s common hatred of each other) makes for reasons for interaction. When the walls are closer together, everyone has to figure out how to relate. Also, in a closed world you can write rules that work perfectly with that specific setting, rather than generic rules that don’t work perfectly with any setting.
Personally I much prefer closed worlds. I like settings that feel real. Infinte mash-up settings don’t feel real to me. I want what everyone is saying and doing to fit together. There’s no reason a closed world couldn’t be used nationally, so long as it inspired enough people to create characters that fit within it. Different locations in the country could be used to represent different parts of the closed world, and be managed by different organisers.

). Most countries where characters come from and games are set are the Mittellande (middle lands), which are a sort of fantastic / medieval Europe. We do have a map and there is not much space on in the middel lands left. If you want to create your own realm