I definitely agree with the silver brushes. I could see them doing them as a "deluxe" set, like BB sometimes does.
Maybe also bring back some original, actually hard to find pigments like Brass, Auburn, Bronze, etc. Originals was such a cop-out in that respect. Also bring back old lipstick colors like Stiletto. It would be fun if they did them in the older style packaging, but with a twist, like a hint of silver on them somewhere.
Betsey Johnson would be adorable! I'd still go ga-ga over a Marc Jacobs color collection, I absolutely LOVE how his runway models look for his shows.
ETA: It also really makes a lot of fiscal sense to bring back things that people price gouge on eBay; the customers are appreciative and likely to buy more, and that money is going into their pockets, not someone's on eBay who hoarded a product from several years ago. The higher ups at MAC have to know that some items like Pleasureflush, Mothbrown (before Barbie), Judy Blue e/s, get into the three figure range on there.