As much as I like the series, there was a lot of missed potential. So I will be naming One for every gen.
Gen 1: More story. It isn't completely void of story, and the game does a good job of stretching it out far enough so you can always discover something new, but adding a little more lore to the games would make it even better
Gen 2: More legendary events: Why did the three legendary beasts just became roaming legendaries, all three of them had much more story and lore then other legendaries. At least suicune HAD something, the other two was left out
Gen 3: More rival battles. You fight may/brenden a lot of times before your third gym badge, and then they just leave. And wally needs more battles too. I still don`t get why people seem to like Wally so much, you see him like, twice in the games.
Gen 4: more direction. The amount of time I spent running around because I didn't know what to do is unbelievable. They could have at least hinted that you should go to mount cornet in the beginning, but literally none tells you too. Cynthia is really useful in these games, sometimes,she tells you where the next town might be (albeit she gives you a task but still useful)
Gen 5: too much story. Oh boy, I feel like someone is going to attack me. Its as if the game took every aspect of it and somehow link it to the plot. I meet up with either the rival, or the evil team every five seconds. The plot was not bad per say, but the game could use more freedom. The only time I felt like I can truly explore the region was when I finished the game. Luckily, B2W2 fixed these problems, so It was much more balanced compared to the former
Gen 6: the difficulty. that is all
I have not play SM and I have foggy memories USUM I might need to replay it
Gen 8: Also the plot. This time, it was completely void of it. When I first got started I thought this was gonna be a great game, They told you about the story. At the beginning. Sonia literally told you everything then dipped. I feel like if they stretched it out, not too far, but far enough for you too feel invested, the story could have been great. take one of my favorite games for example, legend of zelda breath of the wild. They told you the basics at the beginning,but they didn't tell you the full story, unlike sword and shield. You go around finding memories, talk to NPCs, and even do sidequests. If sword and shield made some of the details of what happened and scattered it around, it would definitely work, especially including the wild area.
Note: these are my opinions. feel free to tell me yours