A huge sword!
Make a big fantasy or anime style two handed sword. A great sword, or zweihander that isn't just long, but bulky, in all the right places! (specifically the blade) Something similar in shape to the thinkgeek titan sword(sadly the thinkgeek one isn't strong enough to stand up to larp) would be awesome! I for one would drool over a greatsword like that....
Sounds like the idea theme you want is a Final Fantasy concept. I can already guess from the idea of the size, for the cost of foam this sucker would be in the 300+ USD for the cost of material alone. But to help with MODEL suggetsions. : Auron's Katana from Final Fantasy X; Cloud Strife's Sword frm Final Fantasy; Zabuza's Sword from Naruto; Ashbringer the sword of light that cut down Frostmourne in World of Warcraft; Soul Edge from Soul Calibur; as a list of Blades and Hilts.
The type of sword you're talking about is called a Zanbato. It's the style of swords from japanese manga and fiction. Like Zabuza's sword from Naruto is a Zanbato. It's usually as tall or taller than the wielder, and has a really wide blade ( edge to edge ). A larp zanbato would be so badass! Usually if someone wants one, they have to make a crappy looking boffer. A Calimacil one though could redefine the whole concept. Obviously it couldn't be the size of some of the really long anime swords, but a 6 foot tall one might work; it would be the same size as the double sword of Kira. The one from thinkgeek looks to have sold extremely well. Imagine how a real larp sword from the masters of larp would sell... Calimacil, you'd be missing out on a ton of sales if you pass this up. In case the good people at Calimacil have not seen a zanbato before, here is an example of what one could look like:
And here is a picture with people for reference to the size/width
i was sooo bummed when i saw that the think geek sword wasn't a boffer. instead they gave it a lightt up gimmik. Sooo dumb.