I think all the Starters should get Megas to level it out and make some of them more competitively viable (*coughJohtostarterscough*). I have a feeling that will wait, though.

Gallade, because Garvedoir got one.

Miltoic, because Gyarados got one.

Ninetales. I just think a Fire/ Ghost mega would be kinda cool. On the fence though; if it didn't happen I wouldn't be disappointed. And they'd have to give Arcanine a Mega (that would be neat, another Fire/ Dragon perhaps).

All the "pseudo-legendaries" because Garchomp and Tyranitar have one.

Sharpedo. It needs a boost, namely more Speed (289 max? Come on, it looks like it would be a speed demon!)

Some Gen 6 Pokemon (nothing really comes to mind, it's just odd none got one).

The "Mewtwo"s of each gen (Rayquaza, Lugia, Ho-Oh, Dialga, etc), since Mewtwo got..two.

Several "non-evolving" Pokemon that are kinda forgettable. Kecleon and...many others.