I personally enjoyed XY more, mainly due to it's innovations, like the 3d graphics, fairy type, mega evolutions, being able to interact with your pokémon outside of battles through Pokémon Amie, easier EV training and breeding, being able to rest at the battle facility without being forced into 7 battles, prolly the best berry farming feature of the series, having the first large scale cities like Lumiose... I also liked how much variety of Pokémon it had and how you could easily build your team with any pokémon you liked, which is the complete opposite of BDSP.
But yeah, both the story and postgame were severely lacking, not to mention it was far too easy, even the Gym leaders and E4' teams were atrocious. Sadly some of this stuff could have been easily fixed, especially the lack of difficulty by reducing more the exp that non battling pokémon were receiving aswell as giving the important trainers better teams, including mega evos for the more advanced ones. Viola could have had a lvl 10 Surskit with better def/hp to last just a bit more, a lvl 12 Heracross (just to make him mega evolve on rematches) and a lvl 14 Vivillon with better moveset like Gust, Struggle Bug, Infection and Poison/Stun Spore. Grant could have gained a lvl 20 Carbink to support his lvl 25 Amaura and Tyrunt with moves such as Reflect or Light Screen (alter its movepool if needed) so they wouldn't go down as easily (especially Amaura with it's many weaknesses). Korrina should have used Lucario instead of a weakass Mienfoo during the gym match and she should have gone for a Pangoro instead of Machoke to counter at least a few psychic types. Since you unlocked megas by that point, Ramos could've greatly benefited from a Mega Venasuar while exchanging his useless Weepingbell for something that could actually counter fire types even if just a little bit like possibly Ludicolo. As for Clemont, he should have gained a Mega Manectric, maybe exchanged one of his pokémon for W-Rotom or Magnezone, since they are stronger then emolga/magneton and fit the gym theme, but still can counter ground types since one has levitate and the other has a move that gives immunity to it while also having Sturdy to survive a possible OHKO. Valerie's Mawile should be able to mega evolve otherwise it serves no purpose and she should have had something like an Azumarill to help protect the Mawile from fire types. Olympia could have a Mega Medichamp and exchanged one of her pokémon for Malamar since it's half dark type. Wulfric could've used a M-Abomasnow, a bulky water type for the fire types like Walrein and an Aurorus to set up Aurora (that should have been possible from the get go), while keeping Cryogonal and Avalugg.
For the E4, they *must* have more than 4 mons with at least one of them being able to mega evolve. Like, give a m-houndoom, talonflame, chandelure, pyroar and torkoal or delphox to Malva. For Drasna, a m-garchomp, noivern, dragalge, tyrantrum and druddigon. For Wikstrom, a Klefki, Aegislash, Probopass, Escavelier/Scizor and M-Aggron. Meanwhile, Siebold could've used a Clawitzer, Barbaracle, Starmie and Mega Gyarados (i know Lysandre has one already, but i reckon there are better megas for him if we change his team a bit, like maybe m-scizor or charizard x/y depending on the version). As for the Champion, there is little we can do to save the awful synergy due to the Kalos-only pokémon theme, so powering up their lvls, stats and moves would be the only solution.
Overall, all of XY's major opponents could all benefit from higher levels, IVs and EVs, possibly even having hold items too. Since they have so many advantages in this game like the new exp. share, the affection and countless mega stones to use, boosting the opponents up this much really wouldn't really be too much.
Now, at least when it came to distributing the EVs and IVs for the gym leaders, that is something BDSP did much better, even if some of the teams weren't exactly the greatest. The exp. share is also just a little more balanced, making the level differences not as bad, at least in certain parts.
As for XY story, they truly should have gotten rid of that beauty motif, as it made absolutely no sense. Overall, Lysandre's goal is as underdeveloped as it gets. I feel like they could have leaned more towards the fact that he truly admires Pokémon but he hates that they are being used as tools for humans' selfish intentions, therefore, he is willing to create a world without them. That way the plot would focus more on why pokémon are so important to us and why we should protect them just like they protect us, which is a theme that fits the series' narrative much better than simply human genocide. It's also something that we have witnessed a lot more throughout the series, with each evil team using legendaries for their own selfish reasons and causing destruction all around them. That is something we have seen for years, while Lysandre's bad interactions with humans is not really something that got explored, not even on it's own games.
Speaking of which, i would not have AZ as ancient giant immortal but still as someone who used Pokémon to his own benefit and caused people like Lysandre to suffer. It would be better if AZ had imprisoned Xerneas and used his power to ressucitate his pokémon creating a disturbance between the balance of life and death while causing Yveltal to go rampage, destroying whole villages, like the one where Lysandre possibly lived. Maybe he could have taken Lysandre under his wing after feeling guilty for the destruction that the ressurection caused. That would explain why Lysandre knew so much about what AZ had done and why Lysandre is so against beings that powerful existing. Meanwhile, the Ultimate Weapon could have been created to suck life out of every pokémon regardless of where they are instead of simply throwing a beam that erases both human and pokémon. After all, it would make no sense for him to do all of that to protect humans from themselves when he is already killing a bunch of them.
It would also be nice for the rivals to have character arcs. Like, maybe make Trevor come out of his shell a bit more and have him act more proactive, less shy. Make Tierno understand that he needs to put more effort into training his team. Make Shauna be less superficial, slowly growing to care more and more about helping pokémon. For the opposite gender counterpart, i honestly felt like they were a bit unnecessary, only serving as a tutorial and mega absol exposition, when any other rival could have done that. If it were up to me, i would just get rid of them and have Tierno choose the starter that is strong against you. They could all have a mega aswell. At least that would make them a bit less of a pushover.
But i gotta say, BDSP made me realize how much Platinum's small addittions greatly benefited the storyline. Sometimes characters literally appear out of nowhere, spew 2 phrases with no emoction whatsoever and disappear without explaining anything. Barry and Cyrus are the one that suffers from this the most. Even with just a few more lines and a single new important character (Looker), Platinum was able to make the characters more real and explore better the motivations behind their actions. Overall, BDSP story is not as bad as XY, but it's far from being on par with the third version or even enjoyable in my opinion.
For the postgame, i feel like at least 2 other locations and battle facilities would have made it way better. Couriway Town has a railway that leads absolutely nowhere and Coumarine has a bunch of boats that you cannot sail. Why not take advantaged of them to take you somewhere else? One of these places could have had a rental mode like the Battle Factory or Trainer Agency, while the other could have had a exploration mode like the Pyramid from Emerald and the Whitetree/Blacktower from BW or even a tournament mode where a few previous characters would appear with megas, maybe not as many as the PWC, but at least a few. Either would be fine. It would also be fine if beating their leader would given you a stone.
The rewards given by the other sidequests should've also been improved. Chatelaines should've had rare megas (maybe the exclusive counterparts) and awarded you with their stones for beating them. In the Looker Bureau episode, Essentia and Xerossic should have offered a much harder battle and Malva could have given us a Houndoominite to award us for helping her to clear her name. Unlocking all of the stones should have allowed you to discover a bit more about their lore through Sycamore, even if just to tease a bit of what you would eventually find out in ORAS. Certainly that would be better than simply waiting the time locked events (or changing the clock) just to find them on the ground with absolutely no challenge nor explanation as to why they even exist in the first place. Sure, they could still keep a few of them spread out through the region and easily obtainable as long as you explore, but i feel like we don't need to give that many stones that way when permanent good TMs motivate players to explore just aswell.
In regards to BDSP postgame, i wouldn't exactly say it's much better. It only has a single facility with less modes and freedom than the Maison. The postgame villain sidequest (if we can even call it that) isn't exactly as elaborate nor moving as the Looker Bureau. They do have a few new areas to explore, but they are barren without Platinum improvements.
However, i will say this, they definetly have way better postgame battles to grind and more legendaries to catch than XY. It's just a little sad that the legendaries don't offer as much of a challenge to catch like they used to, other than finding some of the necessary items on the Underground.
Meanwhile, GF could easily fixed XY's problem by giving some of ORAS legendaries to XY, since ORAS already had soooo many.