I know, like how Takanuva was a Nuva rehash, or Lhikan was a Metru rehash, or the Toa Hagah were Metru rehashes... Shall I go on?
Those were one-shot sets, not entire lines of six. When Takanuva was created, he obviously has to take the Toa design of the latest line of Toa, being the Nuva at the time, since TLC doesn't make entirely new Toa canister designs just for Titans and special editions. Same applies to Lhikan and the Hagah.
The Nuva and Mahri had semi-new designs. The Nuva had a brand-new leg pieces, new armor. It was pretty similar to the Mata, but different enough to make me want them. The Mahri had some new designs. Jaller and Matoro both used the Inika back in new ways, and Kongu had an entirely new design, which I liked except for two things: It was too thin, and I don't like it used on Kongu (but that's another can of worms).