XY is literally the embodiment of missed opportunities.
* Amourshipping: Reached status quo early, and then went nowhere until the last episode.
* Pokemon Racing: Infinitely more interesting goal than Waifu Wars
* Clemont and Blaziken Mask/Meyer: No Lumiose City arc where Clemont rescues Blaziken Mask, realizes his father, and vows to be the next B.M. (fulfilling his bravery goal)
* Gangster Malamar: Where tf did they go?!??
* Goodra Vietnam PTSD arc: Resolved too quickly, and without much effort from Ash. Could have been something on par with Ash/Charizard and Ash/Infernape.
* Continuity: How do you just forget about continuity? Even BW, with its egregious 'reset' (that was thankfully gone in XY and SM it seems), had continuity.