I'm surprised to see that someone has the same theory about Termina and the Twilight realm as me...
Well...
I read the article about the "dark tribe" in Zelda wiki, wich may be related to that theory:
http://www.zeldawiki.org/Dark_Tribe
It is about a "dark tribe" wich may be the Twili, because it is said to have been banished ine another dimension via a mirror, after invading Hyrule (FSA). Just read the article to know more about the connection between them, this is convicing.
In MM, the Happy Mask Salesman describes the people which created the mask, here is the quote:
The mask that was stolen from me... It is called Majora's Mask. It is an accursed item from legend that is said to have been used by an ancient tribe in its hexing rituals... The ancient ones...sealed the mask in shadow forever, preventing its misuse. But now, that tribe from the legend has vanished...
If we follow those theories, the "dark tribe" described in FSA became the Twili and they also are the "ancient tribe" wich created the Majora's mask.
Of course the "ancient tribe" may be a different one... But now we have these similarities between the Fused Shadows and the Majora's mask. First, they are EVIL MASKS, which is kinda uncommon. And then they have the same eye design...
There is a lot of other questions... Why did the tribe (possibly the Twili) created the mask for? Are they the "ancients" who sealed it? (the ancients may be the light spirits from TP after all) Did the tribe vanished because they were sealed by a mirror?
So, we now have elements which suppose that the Twili created Majora's mask... Now, why should Termina be the Twilight Realm? It is not said that the ancient tribe that created the mask was from Termina. But I remember what Midna says about it: There is a Twili legend which says that an animal will come to save them, or something like that. In both MM and TP, Link can change himself into different creatures, and then manage to save the alternate dimension...
Well, I personnaly think that Majora's Mask and the Fused Shadow are connected, and I also think that Termina and the Twilight Realm are the same place. I think studying Midna's quote about the Realm may be entertaining, she describe it very often.