An Apple TV needs to be around $100 ideally lower. You can see a more fully functioned vs ATV cheap Mini with a form factor similar to the ATV but its going to cost $300+. You cannot in my view merge the Mini and ATV as it cannot be cheap enough.
If a new Mac Mini was small enough then I suppose they could use the same manufacturing for both, except that the Apple TV would continue to use ARM and fixed specs for affordability, while the Mac Mini (in brushed aluminium to differentiate it) has the better specs at a higher price, i.e - they merge the manufacturing, not the product as-such.
Personally though I don't see the Mac Mini going as small as an Apple TV just yet; it would only create problems, and it's not like the Mac Mini isn't already pretty small. I mean, IMO the Intel NUC size is too small; don't get me wrong, I'm loving the tiny motherboards and cool running chips they use, but Apple could easily use similar components in a slightly bigger case, which gives them the ability to run better specs with no, or modest, cooling, keep the power supply internal and keep a good level of connectivity. So while the Mac Mini could be a bit smaller, I don't think making it so much smaller really benefits it; I don't know many people who are in the market for the smallest ever desktop computer, they just want one that isn't
too big, i.e - unless you're a gamer or need a workstation with tons of expansion cards and hard drives, then there really isn't much need for you to have a bulky tower anymore.
I do hope Apple will return some of the media capabilities that the Mac Mini has lost though; Front Row was annoying to lose, and considering the Mac Mini has an HDMI port as standard it really should be able to take the place of an Apple TV out of the box, as you're paying enough for it.
