I think that the main token which should Eth holders a bit worried is EOS. if they do what they say they can then it might just destroy the dominance which Ethereum has in the smart contract platform.
Till now they appear all talks and No Real world competition to Ethereum.
I still believe that there is no other alt in league with ETH who have any thread to there dominance.
NEO is very well the next one after ETH, EOS, I am not even counting it as a challenge at the moment.