I think that it'd be cool to have two or more colors mixed together (like a tie-dye-sort-of effect) would be great. Like brown and white, or gold sparkles, or black and red.
The goal wouldn't be to have them mix, but sorta swirl together. Apropos of Alembic's roots?
Could you do a metal flake, or would that freak out the magnets? You could use a non-ferrous metal.
Alternately, what about patterns-- put a shape in the bottom of the mold, and pour another color over it?
Or how about lasers? Everybody needs more lasers.
Bradley
PS- the last suggestion was a joke, but for the rest I'm really serious.