Halo 5: Guardians if that counts 💀

In Fire Emblem Fates you can scan amiibos of Marth, Ike, Robin and Lucina which unlocks a battle with them, and if you win they are recruited as playable units (non-canon, on-amiibo DLC).

Considering MegaTen games are here, I think this fits too.

In megaman maverick hunter x, you fight X and Zero at the end of Vile's campaign

Arkham Knight Spoilers ||The game ends with you fighting batman as the joker (well a psychological construct of the joker within Bruce's mind) but close enough||

pretty much every Tales game should be on here tbh.

FFXIV, but im sure other FFs as well and if you count legendaries as bosses ORAS and im sure most of the series would also fit

sonic adventure 2 should be here

in gta san andreas you dont exactly fight but you race against claude, the protagonist of gta 3, in the mission "farewell, my love..."

Sonic 06

@Lot0 The first game in the Coven series is Witch and the Hundred Knight, you fight Metallica in the postgame

@Apocynadeae Thank you very much!

The Last Of Us II kinda counts (Ellie is a major mid-game boss and she's playable during long stretches of the first game and is narratively the main character of both games)

Also STALKER: Clear Sky

one more - Stranger of Paradise: Final Fantasy Origin is kind of a funny case. the Different Future DLC has you fight Jack (protagonist of the base game) while playing as the Warrior of Light (protagonist of FF1). but the Trials of the Dragon King DLC has you fight the Warrior of Light while playing as Jack. so it kinda goes both ways.

Touhou 8 Imperishable Night

Also, for very complicated and spoiler-heavy reasons, you can add Void Stranger

In Xenoblade Chronicles 3 Future Redemmed you fight both Rex and Shulk as part of a story mission.

The parallel canon boss in splatoon 3 side order is the memverse's security system, which is based on agent 4 from splatoon 2's singleplayer

in King's Field III you fight the first one's protagonist

Where is Yakuza 6? Akiyama is like one of the first bosses you fight in the game

I think it's pretty wild there's not a single Metal Gear game on this list, though technically..it's accurate. Unless you somehow warp the meaning of 'previous game' to mean 'previous on the timeline'. I'm almost tempted to suggest MGSV: TPP since you fight Liquid Snake, who IS identical (more or less) to Solid Snake on the genetic level :P Though lets not go too weird..

Would fighting Venom Snake and Big Boss in Metal Gear and Metal Gear 2: Solid Snake on the MSX count considering MGS3 and MGSV didn't come out til waaaaaay later

wait cassette beasts has a previous game? i played a bit of it like a year ago so i didn't get that far but im still curious

Every playable character from DoDonPachi DaiOuJou returns as a boss in DoDonPachi Resurrection :
Exy (stage 2 boss), Leinyan (stage 4 boss), and Shotia (stage 5 boss)

In GTAV, Trevor murders the player character from one of the GTAIV expansions, but it's in a cutscene. Probably doesn't count.

Fire Emblem Radiant Dawn is a game where you fight Ike, the protagonist of the last game in the series Path of Radiance

Fire Emblem Awakening has a bunch of DLC in which you fight various previous protagonists, but most notably a DLC called Xenologue: Champions of Yore 3 has you fight literally every previous protagonist on the same map.