Well, I, for once, would like the underwhelming pokemon, those who can really use a boost in order to shine, to get a Mega instead of the popular ones. Mawile, Absol, Ampheros, for example: all were a step in the right way. Gengar, which is already OP? Not so much... Garchomp, Scissor and Tyranitar? It seems there are more Pokemon who got mega because their popularity then those who can actually use it.
So I say, of the Hoenn Pokemon: Give Mighthyena a Mega. That is my first choice. I guess Zangoose and Seviper (like Heracross and Pinsir) are good, obvious, choices, though not the best ones since they are kinda already good at what they do. How about Tropius and Chimecho? These pokemon always fell under the chair and Tropius is a standalone Pokemon practically begging to being useful. And how about Mega Glaile? This guy was discarded by Nintendo ever since his better female counterpart came out.
Also, I think Jynx doesn't need Mega. She needs an evolution. A regular, good ol' evolution. That way she would really be like her fellow buddies, Electrabazz and Magmar. Giving Megas everywhere instead of evolutions will get old fast, and isn't really necessary. The same goes for handling Pokemon like Luvdisc (maybe Nintendo should finally do the obvious and make an evolution chain between this one and Alamomola).