Endgame Plus

Bonus content in a Video Game (and RPGs in particular) that is available during the final chapter, but does not become unlocked until having defeated the Final Boss and watched the credits roll. These games will often prompt the player to update their save file after the end credits, often displaying a special icon with the file to indicate the game has been cleared.

This differs from a Playable Epilogue in that the Playable Epilogue is set after the final chapter has concluded, and acknowledges that you've saved the world; an Endgame Plus returns the player to the world as it looked during the final chapter, with the Big Bad still waiting for his ultimate plan is to come to its final fruition, and the player standing just before the Point of No Return or outside The Very Definitely Final Dungeon to throw that critical Spanner in the Works and Save the World.

This is distinct from games that unlock bonus content via an "Extras" option on its frontend menu; here, the bonus content is something accessible in-universe, like a bonus shop, minigame, or dungeon...or even just clothes. The exact bonuses may or may not be immediately obvious when the player reloads their completed save file -- but hey, Take Your Time....

Note that games featuring a level select screen cannot exemplify this easily, because they allow the player to re-play any segment of the game at any time.

A form of Extended Gameplay. See also New Game+, where reloading the completed save file restarts the entire game over with added bonus features. Both of them are subtropes of Post End Game Content.

Action Games

 * A very early attempt at this was Intellivision's Tron: Solar Sailer. Once you defeated the datastream puzzle, you could either cash out and win or go double or nothing at level "insanity" for bragging rights.

Action Adventure Games

 * Defeating the Final Boss in The Legend of Zelda the Minish Cap reveals no obvious bonuses, but nonetheless unlocks a few collectible figurines for purchase, including those of the Final Boss. It also enables the player to acquire the Mirror Shield.

RPG -- Eastern

 * After the credits roll in Breath of Fire IV the game creates a "Clear Game" file, which resumes your saved game with an added bonus shop (with rare items) run by two characters from Breath of Fire III. The Final Boss is still waiting for you to challenge him (and any items stolen off him during the final battle are yours to keep).
 * Chrono Trigger (the DS version) allows you to access the dimensional vortexes. Beating all three of these gives you access to the True Final Boss, which is an "immature" form of the final boss in Chrono Cross.
 * A staple of the Dragon Quest series is to unlock a Bonus Dungeon or two upon beating the final boss; beating it is necessary to get the Golden Ending in the games that have one.
 * Variation in Earthbound: You lose the ability to save after the Final Boss, the idea being that you can reload and beat the Final Boss again.
 * Golden Sun Dark Dawn has Crossbone Isle accessible once you beat the final boss. It contains rare items, powerful monsters, and the ultimate Bonus Boss, Dullahan.
 * Final Fantasy XIII unlocks the last layer of the Crystarium for beating the final boss, but story still rewinds to the point before your confrontation with him. You can then backtrace your steps to the beginning of the Very Definitely Final Dungeon and from there, to the locations you visited in chapters 11 and 12. This was probably done because.
 * Final Fantasy XIII-2 allows you to "Lock" eras and replay the story from there (Required for One Hundred Percent Completion in some instances). Plus, you unlock an era after the final boss (although the era in question only really for fun).
 * Kingdom Hearts Recoded gives you access to a number of things, including new shop items, the last chunk of your stat grid, and the Bonus Dungeon. You can replay everything anyway, but the game does count you as being at the end of the last world.
 * "Kingdom Hearts Re: Chain of Memories has a complex version of this, where after you finish Sora's story you unlock Riku's, and after finishing Riku's you can get two new cards, an invincibility card, and a premium-reloading one.
 * Beating the final boss in MS Saga: A New Dawn unlocks the Bonus Dungeon. The game even provides a small Lampshade Hanging as The Hero complains that he's about five seconds away from the ultimate battle to save the world when Mission Control phones in and orders him to go conquer the Bonus Dungeon instead.
 * While the mainstream Pokémon games generally utilize a Playable Epilogue, in the original Pokémon Red and Blue titles, the rewards for becoming the Pokemon League Champion were limited to a "Hall of Fame" option on the PC and access to one Bonus Dungeon near Cerulean City; no one else even acknowledged the fact that the player beat the Elite Four and The Rival.
 * Defeating the Final Boss in Rogue Galaxy adds the game's Bonus Dungeon to the Galaxy Map; everything else (including the recap) is still as it was before defeating the Final Boss. An NPC within said dungeon even comments that the player has already beaten the game.
 * In Final Fantasy Tactics a 2, beating the game unlocked certain missions and made is possible to unlock the last stage of Brightmoon Tor.
 * The Mega Man Battle Network series does this in every game, following the trope description almost exactly, with the single exception of 4. It also carried over to its pseudo-sequel series, Mega Man Star Force. One annoying thing about this though, is that even though you beat the game? The crisis music (which plays in the regular areas -all the time- during the last part of each game) doesn't stop, besides in 3.
 * The Bonus Dungeons in the Game Boy Advance versions of Final Fantasy IV, Final Fantasy V and Final Fantasy VI become available after beating their respective Final Bosses once.
 * Mega Man X Command Mission lets you replay the final chapter over and over again, which allows you to power level your characters and save any items you find each time. You can also revisit any level in the game as well as fully explore your headquarters; in fact, completing the game at least once is a requirement for facing Ninetails.
 * The Game Boy Advance versions of Final Fantasy IV, Final Fantasy V, and Final Fantasy VI all have dungeons that open up after you beat the final boss. Within each dungeon is, among other things, the most powerful creatures in the game and the ultimate weapons for the characters. All three games allow you to defeat the final boss again with whatever new equipment and levels you gain from these as well.
 * Final Fantasy IV not only opens up a bonus dungeon found on the surface of the moon, but allows the player to switch out the characters in his party (which up until that point had been locked in place as part of the story) and lets you go back and defeat the final boss using entirely new strategies due to the differences in the various characters. The bonus dungeon itself features ramped-up versions of Rydia's monster summons, who you have to defeat to use, and an even more powerful version of the final boss, which also leads to the credits if you beat it.
 * Final Fantasy V has a bonus dungeon that contains four brand new job classes never featured in previous versions to use and master. The classes themselves are quite powerful, and the abilities you earn from mastering them can be mix and matched with other job combinations, leading to some truly devastating combos against the final boss. For example, the Gladiator class has an ability that can result in 1 of three possibilities. You have a 1 in 3 chance of missing the target, 1 in 3 chance of dealing 3333 damage, and a 1 in 3 chance of hitting for 9999 damage regardless of the enemy's defenses.
 * Final Fantasy VI unlocks the Dragon's Den, which features ultimate weapons for all the characters, and the Czar Dragon, the most powerful enemy in the game. You have to split into 3 parties and switch between them to cover the various traps and puzzles. This is also a minor case of Guide Dang It, as the dragons inside can be weak or strong against very specific abilities that only certain characters can have, so you need to strategize and choose your parties carefully.
 * When you defeat the final boss of the story of the .hack GU Games, you open up the legendary Forest of Pain, a special event dungeon containing 100 floors of the strongest enemies in the game. According to the story, only one player in the entire game had ever been able to make it to the 100th floor. Just reaching the 50th floor will reintroduce Haseo to his friend and former guildmate, who you can then invite to your party. Due to the simulated MMORPG nature of the game, the game never truly ends, and you also unlock special bonus relationship endings if you propose to a character who's relationship is high enough. Save Scumming is required to actually see them for every character though, as you can only do it once.

Platformers

 * Ratchet and Clank A Crack In Time lets you finally put an end to Lord Vorselon's tyranny, after collecting the rest of the Zoni, of course. At least when you defeat him you get to explore the Insomniac Museum.
 * Mega Man ZX (in normal or hard mode) allows you to get an extra armor after defeating the Bonus Boss.

Simulation Games

 * Harvest Moon A(nother) Wonderful Life (Special Edition) has a chapter after the game ends titled Heaven. The game sets things back the way they were in chapter 5 and the chapter doesn't end, unless you decide to start a new game.