it will take a fairly large breadboard to get enough RAM for a 128KB of RAM. But it could be done, I think.
on my wishlist is a (<10$) device - shield , I2C, or single pin ? - that will generate true random bits.
To reply the original wish list question, I personally believe that he principle limitation of the Arduino platform is the lack of RAM. The CPU is amply fast for all sorts of tasks -- definitely way faster than a 2Mhz 6809 or that I used to develop full-on operating systems for. Just having a decent amount of memory would alleviate so many of the constraints. For example "enough" memory would enable management of multiple thread stacks and the implementation of a good flexible thread library. Even 16K would change things a lot. 64K or 128K would be great.
Just a small detail note, with 128k we're talking about B&W, which is fine by me.