sequels with interesting secrets if you have data from the first game on your memory card

in shin megami tensei if...'s playstation version, if you've played other atlus games, you can get bonuses from this one npc if he sees the save files on the memory card. usually it's in the form of a skill given to the partner character or a boost in a specific stat, but it varies on the save file the game is from.

also with the persona 2, if you renamed tatsuya in innocent sin, in eternal punishment he keeps whatever last name you gave him and it applies to katsuya too ^^!

Pokémon did something like this recently. 

In Pokémon Brilliant Diamond and Shining Pearl, you get a special item if you beat Arceus in Pokémon Legends: Arceus (which takes some time, as you need to catch at least one of each Pokémon you can get in that game, then beat Arceus in battle). 

The item in question is the Azure Flute, which was originally intended to be distributed via Mystery Gift in the original Diamond and Pearl, but was scrapped. However, not only did they bring the item back, it works the same way the item was originally intended to work, allowing you to encounter and catch Arceus at Spear Pillar.