sword art online
elf world isn't filler, like previous said, it's part of the manga, and no it wasn't run out of ideas, anyone who has played video games can recognize the significance of the worlds, before it was a levell grind, then it becomes skill aquiring. filler is defined when the anime series makes up whatever the hell they want, because the manga is moving too slow or not fast enough for them to create a episodes, usually it's a 3 or 6 episode "side story" or a flashback to a younger time, or focusing a 3 episode arc on a terciary character. that is filler, and filler is crap, because it's just there to take up air time, even if it was good filler, which is very rare, it's ignore the second the manga has caught up to speed.