Thundercats sucked. What a 3rd rate cartoon...
Obviously you've never seen Thundar the Barbarian. THAT was a 3rd rate cartoon. Thundercats mostly just suffered from one, fairly talent-less, voice actor doing every single female voice (compared to other cartoons of that era). Not only that, she did most of those female voices while impersonating William Shatner
But obviously, if the Masamune can transform from a knife to a sword by "powering up," then it has disturbingly similar traits to the Sword of Thundera. Like the Sword of Thundera, the masamune provides one with "sight beyond sight" in the form of reflecting the inner character of the wielder (and revealing where the entrance to Magus' castle is). Much of the story revolves around the Masamune, just as much of Thundercats revolved around the Sword of Thundera. Liono was an anthropomorph, Frog was an anthropomorph. Magus controlled dark magic, Mum-Ra controlled dark magic.
Masato Kato is clearly a Thundercats fan.