The Intel NUC (
D54250WYK) is roughly 4.5" x 4.5" x 1.5". It's that small because it uses flash storage and the same CPU that's in the Macbook Air (i5-4250U), which is perfect for an HTPC and everyday tasks. It's a great example of the direction Apple needs to go with the next Mini. It wouldn't necessarily have to have a fancy aluminum enclosure with chamfered edges. A quality plastic enclosure in an assortment of nice colors would be fine.
People that want more compute power, for gaming or video encoding, but don't want to spring $3000 for a Mac Pro or $1500 for an all-in-one, Apple should provide them with a separate Mini (larger design, quad core i7's, BTO gpu's, etc). Even if they used all mobile components, like the same ones offered in the iMac's, that's fine.
These two would fill a huge hole in Apple's product line that a single Mini just can't accomplish.