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gen 1's slightly more gritty tajiriverse and down-to-earth setting, no bullshit godmons or anything, just you, your bros, and the world
gen 2's sense of culture and mythos, legendary pokemon having actual legends to them you hear about through the game, making your eventual meet with them that much more impactful
gen 3's exploration, between the two different types of bikes, HMs, diving, there were a lot of ways to get around the map
gen 4's following pokemon, yeah it's a smaller mechanic but it helps you feel that much more attached to your team
gen 5's minigames/side content, having so much to do outside of the main campaign
gen 6's bottom screen, PSS, super training, amie, and dexnav were all kino im their own ways
gen 7's asymmetrical boss fights, surprised it took gamefreak this long to try things like this, though i'm unsure how you'd incorporate it into gym leaders without fanfic tier "starts rain & trick room permanently at the start of the battle" bullshit
falinks
gen 9's overworld battles, was cool to actually see battles take place right wherever you are