Maybe not for 450 keys (!), but I would dearly love to be able to type all the diacritical marks and at least select Unicode ranges (Cyrillic and Greek).
It's been a really low-priority desire (I won't dignify it with "need") and so I haven't even really worked out a layout or even carefully defined what I'd like, but yeah, the Really Big Keyboard would fill at least a small need.
In A.I. War (book by Daniel Keys Moran), Trent uses a "custom 240-point Unicode board" to code in SuperLISP, which originally put the idea in my head. I asked Moran about this. He made it up whole cloth. No such keyboard exists. Of course, we still have ample time to invent one before the time of that story.
If you persuade your operating system to load up keyboard maps with the ISO 9995 common secondary group, you will get all of the combining diacritical marks at least. They're mostly on level 3 of group 2.
