Save Data Bonuses Were AMAZING

Latest version of this I can remember is TOTK automatically carrying your horses over from BOTW

The first one that came to my mind was getting Rosalina in Mario Kart Wii immediately if you had Mario Galaxy save data!

On the DS, if you had a copy of Castlevania Aria of Sorrow in the GBA slot, then booted up it's sequel, Dawn of Sorrow, you'd get a special item, and would see a little doll dancing in one of the shops.

The armored core franchises would almost require you to transfer files from the previous game because of how hard some of the titles would become, making a fresh start not optimal. Best example being Armored core Nexus to Armored Core Last Raven.

In Tekken 3's theater mode you can insert discs of Tekken 1 and Tekken 2 to watch the character endings from those games.

Nickelodeon had this massive cross-saves feature on PS2 around 2005/06, with the games SpongeBob: Lights Camera Pants, Tak: The Great Juju Challenge, Barnyard, and Nicktoons Unite, allowing extra features in each other’s games to be unlocked like bonus costumes in Nicktoons Unite. Thing is, there is almost no footage of the unlockables anywhere online so a full video dissecting that would be pretty neat.

The devs for Legend of Zelda Oracle of Ages & Oracle of Seasons wanted a feature like this, but couldn't because the save data was limited to each cartridge. So they tell you a secret name at the end of each game, and if you give your character this name on the other game, it starts with a different cutscene and changes the story. Stuff like this is so cool! Great video

Monster Rancher was one of my FAVORITE games. We NEED a modernized version of the disc-to-monster feature, it was so much fun trying to figure out what CDs and movies had cool creatures.

the trails series is an excelent example of modern save transfer. they do both save transfer and bonuses, transfer is for a same story arc and bonuses is for older games remasters

Randomly unlocking rosalina in mario kart Wii for having mario galaxy save data as a kid was always a nice surprise

Could name one which blew my mind as a kid. So in Midnight Club (the first one), if have save data for Smuggler's Run, you'll unlock the dune buggy.

Ratchet and clank for the ps2 had a very creative Save Data Bonuses They even was self aware of the joke Discounts, Free Guns and Humor got to love Insomniacs ingenuity.

The ones in the Original Ratchet and Clank Trilogy were always fun The 2nd Game lets you get the Weapons from the 1st Game that are in the 2nd for Free if you bought them The 3rd game has the same for the Weapons of the 2nd Game in the 3rd for Free, but also had one from the 1st Game, in addition of Unlocking the Look Ratchet has in the 1st Game, if you watched the Cutscene where Ratchet is told he'll get the Employee Discount after 2 years, you actually get that Discount

Most memorable one for me was Jak X unlocking ratchet and all the last 3 versions of Jak as racers if you had save data for those games. Was my first experience with that stuff.

Using all the prequel characters in Lego Star Wars II was so satisfying as a kid.

To this day my first and favorite example of this were the Golden Sun games on GBA. In the sequel, about 2/3 of the way through the game the original party joins forces with the adventuring group of the sequel, and if you connected another gba loaded with the first game with the multiplayer link cable it would import them exactly as you left off (which was broken af). If you didn't have the cable you could enter a password with different length "tiers" that would import more information. The lowest was 16 characters, and the highest was 260 for a completely accurate copy

I think one of the coolest examples of save data transferring is with The Suffering and The Suffering: Ties that Bind. The original game had a total of three different endings depending on the protagonist's morality by the end of the game and in The Suffering: Ties that Bind, there are a total of three different openings picking exactly right where your ending left off.

Dead Space 2 would give you the original DS1 plasma cutter if you had saved data from the original.

The last ones I remember seeing in the current era are the bonuses you'd get for beating the demos for Nioh (Mark of the Strong and Mark of the Conqueror), which would unlock cool unique pieces of equipment when the full game came out.

Bardock was automatically unlocked in Budokai 3 if you had save data from Budokai 2 on your memory card. The Ooze and possibly some other games were unlocked in Sonic Mega Collection if you had a Sonic Heroes save.