Starter: A throwback to Pokemon Yellow. Start us off with a Fairy pokemon, instead of the three starter pokemon. Maybe even have that Fairy starter be a new/unique Pokemon.
Eh, I don't know about this... most of the Fairy-types are too cutesy for my tastes. I'd prefer the original three starters, even if this would be a change of pace.
That being said, in Yellow, the three Kanto starters were all obtainable regardless, so...
GTS: We need to be able to see the natures on Pokemon that are being offered for trade. It's important! Also, a clear indication of what original region the Pokemon is from.
And abilities, perhaps? Just to make sure you're getting the one you want.
Otherwise, my wishes for Z/sequels/spin-offs are:
-Zygarde getting a new form. Although that seems all but guaranteed, since it's a usual third game thing. It'll
probably be done as a Mega evolution, since that's the newest form-change-esque thing. I'd also ask for it to gain a better signature move, since Land's Wrath seems a bit lacking, but I'd be okay without one.
-More Pokemon gaining Mega evolutions. How about giving them to more Pokemon that actually
need them, instead of milking for popularity? Where's my starter equality?
On that note, please no more version-exclusive Mega stones, that's ridiculous.
-Different difficulty levels available from the start, with no need to unlock them. Because I myself have never had a problem with the difficulty level for any of the games, but who doesn't like a challenge? The Easy/Normal/Hard modes in Black 2 and White 2 were a step in the right direction, but were a chore to unlock.
-And finally, I'd like to see AZ again. Darn giant hobo and his immortal Floette grew on me, what can I say...
/wall of text no jutsu