If you want to get technical, they were made by the same people (the creators of the series) which seems to me could also explain the design similarities. The only thing similar on the two items is the eye and that particular eye design has been used MANY times in the series. Regardless of it...
Keep in mind that the Happy Mask Salesman apparently knows how to freely travel between Termina and Hyrule without any magic (to our knowledge). There doesn't necessarily need to be some special way to travel to Termina (mirror, skull kid/mask powers) because a (mostly) normal Hylian knows how...
I don't recall anyone ever stating that the Fused Shadow is an evil mask... In fact, it's used by Midna (who is an ally and helps Link save Hyrule), so I fail to see what makes you think that the Fused Shadow is evil..
I completely agree that Majora's Mask has no connection to the Fused Shadow, but you know how much I love to play the Devil's Advocate so in defense of the theory at hand, at the end of MM the HMS said that the Mask's power was gone (meaning it was just an ordinary mask). It's possible that with...
according to the theory the creator of this thread has, that would mean that twilight princess would come before majoras mask on a timeline.
the reason for what i just said is because in order for the twili to make majoras mask then the twili would have to exist during Majoras Mask, but the...