The $10 theory

[Grey Acumen]

I have to admit, the moltov cocktails style isn't really my thing though the broken defibrillator sounds interesting. It sounds like this is designed so that the weapons you use in the game are found and improvised from whatever arena you happen to be in.
In order to conserve Wii Memory, you may want to have it so you can save any purchased levels to an SD card, but in the game, you can switch those levels into "active" state by moving them into the Wii memory. That way you can switch out levels much like Nintendo has sugested people switch out their virtual channels, except this would provide an in game method of doing that. Of course, any game you buy or that is for free should be redownloadable for no extra charge, but letting people store it on their SD cards means they don't need to use bandwidth every time they want to load a level.
Actually you should probably be able to Have levels active even if they're all saved to your SD card. Super Smash Bros Brawl allows you to save any of your custom stages to an SD and also access them directly from those cards, so it should be doable for extra levels for this FPS as well.
One issue about saving the levels to the SD card is that people would likely try copying the data so they could use those levels on a different system, so it'd probably be necessary to implement a online verification for those levels that aren't free, so that the game can check that you have purchased the levels. Of course, this should technically only require a moment, since it only needs to verify that you own the level and not download the entire level.