Ah yes Ultraman, I don't know why that didn't register when Kanji pointed that out. Hell it's not just the colour and size changing but also it's another instance of an alien setting up shop in someone's body, all be it with consent in that case. Yes far better then Ideon.
On the subject of assumption, well thats kind of the point of this thread. Takaya did indeed site those as his main sources but from examining other content, others research and personal experience, development of any concepts is not always that simple. We constantly absorb new information and idea's that we then develop into our own, one way or another. It's all very interesting I feel and applying a little speculative analysis and assumption to Guyver is both interesting and fun. If absolutely nothing else, doing this with other stuff has allowed me to come across other material I was unaware of.
Edit: It's also a good way to find out your ideas are wrong, case in point.
On that note the placement of the Control Metal and Zoacrystals is another thing of intreast. The most obvious is the idea of the third eye mythology and spirituality. It's a concept generally linked with greater awareness along with the ability to see chakra/auras/ the supernatural. While not a true facsimile of these concepts, the Control Metal is the core of the Guyver and the direct link between the host and the armour, providing control, information and protection. Perhaps the closest it gets is with the units ability to connect directly with the Relic ship. The Zoacrystals while not as complex are still the key part of Zoalords power.
Guyver is far from the only one as it has been used in a lot of media, for example Mazinger Z again with the placement of the pildet. For the first piloted mecha, it's control unit is not behind it's eyes or in it's chest, but right in the centre of the forehead.
As well as with it's progenitor Demon Lord Dante, with the human part of the demon (interesting also inspire by Godzilla).
Other more overt references to the third eye can be found in a Japanese's fiction such as Tien from Dragon Ball to as recently as One Piece and the character of Pudding.