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And I suppose Mashima did need to research various forms of dragons for the Dragonslayers. Along with the constellations for Lucy's Stellar Spirit magic. And then the names of the characters I'm sure aren't entirely random. Lucy came from the Beatles' "Lucy in the Sky with Diamonds", Natsu was to continue his naming the heroes after seasons (plus, "summer" is hot, and Natsu is a Salamander...), Happy was from the bluebird of happiness (blue cat who can fly, anyone?), Laxus I'm sure came from the Latin word for "light" (Laxus used lacrima to make himself an Electric Dragonslayer)...
What's interesting about his two main works is that both heroes are cliché in terms of their ability/weapon/etc., but Mashima twisted such and made them both unique. Haru, for example, is a kid who fights with a giant sword. But the sword can change into different forms, each with their own special ability. Then there's Natsu, who's magic is fire. But it isn't just normal fire (look at Totomaru of Phantom Lord's Element 4 for that), it's Dragonslayer fire. To top it off, he can eat fire that isn't his own and replenish energy from it.
I still have so many chapters in Shaman King to go. But it's a bit less than where Gintama's at, not to mention Gintama's still being published.