Hmm... come to think of it, between them, Gen. I and Gen. III have around 73% of the Mega Evolutions that we know of (while only having 40 % of the total amount of Pokémon). Assuming GameFreak hasn't burned all their powder yet (and I see no reason to believe so), I think we're going to see a couple more Mega Evolutions in the months before launch. My money is on something from Unova being next. With Diancie getting a Mega, it seems like the rule of "no late-generation Pokémon unless they're
really popular or have a typing we haven't used yet" is soundly broken, and Gen. V remains the only generation without a Mega Evolution.
With this in mind, I also think we shouldn't expect too many more Gen. III Mega Evolutions. Counting the Latis separately, Gen. III already has the most Mega Evolutions, with 13. I can see the number hitting 15 with Metagross and Salamence, that being the limit until Gen. VII comes around (with "Primal Devolution" circumventing the limit, in the case of Groudon, Kyogre, Rayquaza and possibly 2x Zygarde and/or Xerneas/Yveltal).
I saw this helpful little chart on Pokémemes today, which gives a nice visualization of how dominant Gen. I and III are in the Mega field:
Also, Gen. II seems a little under-represented. Then again, that might be because it's the only of the first three generations not to have Mega Evolutions for its starters. Which raises a few interesting possibilities...