Would another 12-inch MacBook work? Who knows? But it would fit neatly into Apple's current lineup.
Never, ever would I presume to have a better idea than the marketing and design geniuses at Apple, but let me throw this out there anyway. With the MacBook bottoming out at a 13.3-inch screen size and the iPad at about 10 inches, a 12-inch ultraportable would find a niche at Apple.
I can hear Apple designers bristling at the idea already: 12-inch form factors are so passe! Too small! 13.3 inches is the perfect combination of portability and size!
But wait a minute. With today's materials and silicon technology, 12-inch laptops lend themselves to the next grail of laptop portability: less than 2 pounds. With Apple's knack for trend-setting designs, I can envision a 1.5-pound, razor-thin (under 0.7 inches) MacBook that showcases Apple's design prowess.
Of course, it would be priced well above the iPad to avoid any hint of direct competition. But think of the possibilities: a special ultra-low-power Intel processor (Apple and Intel would need to bring down the thermals--the heat generated by the processor--a bit, even below Intel's newest power-efficient processors).
Or, as possibly a better option, imagine this: Take the iPad's basic internals, add a next-generation dual-core Apple A4 processor, tack on a built-in keyboard, and, presto, a very unique clamshell laptop not beholden to Intel (which, I'm sure, is fine with Apple). And Apple could architect an enhanced iPad-style operating system. (With Flash support this time, please.)
Of course, there have been a raft of decent 12-inch (and smaller) laptops in the past from companies like Toshiba, Sony, and Hewlett-Packard. Sony sells the 1.5 pound Vaio X series, and HP has continued to update its popular EliteBook ultraportable line with the 12-inch 2540p. But the Sony design is based on a sluggish Intel Atom processor running an overburdensome Windows 7 operating system. And the HP design is more of a buttoned-down business notebook.
Apple could bring some real panache to the ultraportable laptop market (again), which, I would argue, is getting stale. Would you buy one?