I want low-tier pokemon which cannot (or should not) evolve to receive megas. This is why I was really happy when Ampharos and Kangaskhan got megas. They were both low tier, and had no room for evolution (or shouldn't in the case of the latter, whose appearance is already that of a fully evolved pokemon.) I also want there to be an emphasis on pokemon from earlier generations getting megas, to put them back into the limelight.
Some examples of the type of pokemon I want to receive megas are things like Pidgeot, Poliwrath, Bellossom... all underpowered fully evolved pokemon. Pidgeot could finally live up to its dex entries of flying to speeds of Mach 2 with an ability that allows speed priority on everything. Poliwrath could have an ability that makes Hypnosis 100% accurate. Bellossom could have an ability which prevents locking yourself into Petal Dance, doubles Petal Dance's damage when resisted.
The pokemon I want to receive a mega the most is Cherrim. Am I the only one who finds it very annoying that you have to activate Sunny Day every time, just to see it it in its better-looking appearance? Despite having higher special attack and that both Solarbeam and Weather Ball works well in sun, Flower Gift raises its physical attack. (And before anyone brings up Drought, any tier that allows the ability has no room for Cherrim.)
I want a Mega Stone which automatically switches Cherrim into its Sunny Forme (turns out it was its mega all along! :') ), and which will remedy its ability to be more useful in the sun.