Following your suggestion I slapped together a quick model in SolidWorks to see what it would look like:
3x3x9 fisher full.jpg [ 203.01 KiB | Viewed 541 times ]
...with mechanism and all!
3x3x9 fisher.jpg [ 183.38 KiB | Viewed 541 times ]
I'm sure it would be difficult to solve, because of the higher number of cubies, and that it shapeshifts.
I don't think so. The only thing that shapeshifting adds to the solving challenge is a different perspective. High numbers of cubies doesn't mean that it's difficult to solve, it just means that it takes awhile to solve.