As a meta-example, many players tend to base their Player Character in the game off some other character in fiction, generally known to others they play with. This is a common and generally accepted as fun practice, but there are some characters who tend to get exported much more commonly than others to the point of being seen as cliche. For those, see Overused Copy Cat Character.
The Forgotten Realms setting of Dungeons & Dragons has a few also: King Azoun IV of Cormyr is pretty obviously King Arthur, especially with regard to the whole "Sleeping Sword" thing, which parallels Arthur's "Sleeping King" myth pretty well. Arthur's coat of arms was a golden dragon, Azoun's is a purple dragon, etc. Also, Yamun Khahan was a very obvious expy of Genghis Khan. So when the two of them fought in single combat....
It's implied that Chandra was taught by Jaya herself, going so far as mentioning Jaya's name when recalling one of her lessons.
Magic also does expies of real-world mythology and literature in some of its settings. This is most obvious in Theros, where there are expies of specific gods and heroes as well as expies of the cities of Athens and Sparta, and an expy of the Trojan Horse.
Mecha Vs Kaiju, in its FATE version, gives statblocks for Not Zilla Kaibutsu, Ghidorah-inspired Sanshuseki, Gamera-derived Kameyuu, and Hedokoku, who most seems to resemble Hedorah in being a monster fuelled by pollution. Others are mentioned in backstory, such as the moth monster Senshuga.
Since almost every culture in the Mystara setting is a Fantasy Counterpart Culture, the creators have not been shy about including culturally appropriate Expies such as Manuel of the Plains, a masked vigilante from a Spanish-inspired culture who is an obvious Zorro Expy.
Pokethulhu has many expies (often double-expies) of characters from both of its "parents": Randy Carter (Ash Ketchum/Randolph Carter), Derleth and Bloch of Team Eibon (Jessie and James of Team Rocket), and even Pikathulhu (Pikachu and Cthulhu).
The Ravenloft game setting is swarming with Expy versions of classic characters from Gothic literature. Dracula, Dr. Frankenstein and his creation, Jekyll & Hyde, and so on: they're all there, sans serial numbers, disqualified from being Captain Ersatz only because the originals' copyrights all expired ages ago.
In Strike Legion, just about everything is an expy of something from another sci-fi or fantasy setting. The game not only shamelessly rips off other settings, but then it takes those same things and turns them Up to Eleven.
Most of the Icons of Thirteenth Age embody fairly standard RPG character tropes like the Emperor, the Archmage, the High Druid and so on, but the Great Gold Wyrm is pretty clearly Bahamut from Dungeons & Dragons with a different scale colour.
The factions in Warhammer 40,000 were originally little more than faction-level Expies for Warhammer Fantasy factions. Although they've tried to tone this down a bit — for instance, by phasing out the Squats without explanation, then finally saying they were just eaten by the Tyranids — there's still obvious parallels. Empire —> Imperium, High Elves —> Eldar, Dark Elves —> Dark Eldar, Undead —> Necrons, Chaos —> Chaos Space Marines, Greenskins —> Orkz, Dwarves —> Demiurg (a Tau-allied race), and to a lesser extent, the Tau in general. The Tyranids are an exception — there's been some awkward attempts to equate them with the Lizardmen, but it doesn't really work out.
Especially since the Slann are more or less the Warhammer 40K equalient to the lizardmen, while the 'nids are more or less expies from Starship Troopers.
An example of a smaller background race in the setting is the Hrud, who are practically expies of the Skaven in many ways. At least as far as the fluff regarding them gives away. Both are rat-like humanoids who live in sewers in various places, and both use warp-plasma based technology and are scavengers, plus the addition of similar social habits. It is highly unlikely that they will become any kind of major faction in the tabletop, however, since Games Workshop are heavily opposed to the idea of having Warhammer 40000 as simply Warhammer in space, due to the removal of the Squats, the original Dwarf expies.
As of its printing, Games Workshop has some funky canon out now that makes them look like weird spine-like jellyfish people (which is actuallyquite plausible when you...). Although they still have some Skaven elements (photophobia, cavedwelling, Warpstone usage), they also have some new canon junk like a God Pantheon, a curbstomped god who was shattered into psychic shadow spheres called Umbra via Slannesh (who else), and some weird time-distortion, warp-radiation field of narminess which they constantly radiate about themselves.
Also, the Imperial Arbitrators were recycled from an abandoned Judge Dredd tabletop game.
Nearly everything in W 40 K is an expy of something from another setting. Aliens vs Starship Troopers vs Colonial Marines vs Terminator vs Robotech vs Elves vs Orks vs Demons etc... While the original game was largely Warhammer Fantasy IN SPACE, the current iterations of W40k are closer to "sci-fi trope deathmatch" with the awesome and grimdark turned up to 11.
While being essentially the tomb lords from Warhammer Fantasy, the necrons are also clearly terminator expys. Rules like "I'll be back" are pretty obvious clues.
The tyranids are pretty clearly expys of both the Aliens and bugs from the Starship Troopers movies, depending upon the subsetting.
For example, genestealers and general indoor fighting of tyranids is heavily based on the Aliens film. Space Hulk was basically Aliens, the board game.
While the outdoors stuff, especially the perennial matchup of imperial guard vs tyranids is straight up Starship Troopers, the movie.
Khorne. A War God who's favoured warriors are called berserkers, his bloodletter daemons are described as "blessed dead" and is associated with the exhilaration of battle? Not to mention in Warhammer Fantasy, he commands daemons who collect the souls of the dead, his realm is named "Kreignihalla", and his symbol looks like a Valknut◊? Hmmmm. Sounds familiar.
Ghazghkull Thraka's early history is based on Adolf Hitler's: he was a lowly Goff (the Ork clan known as being militaristic and serious) boy who got severely wounded and then had visions of Orks conquering the Galaxy, just as Hitler was just a German (stereotypically known as militaristic and serious) corporal in ww1, got severely wounded and... well you know the rest.
On the other hand is Lord Castellan Ursarkar E. Creed of Cadia, who is Winston Churchill, down to the cigar, with some elements of General Patton thrown in. Fittingly, Cadia is basically a Fantasy Counterpart Culture for Blitz-era Britain.
Trazyn the Infinite is basically Doctor Doom if he were an alien space robot.
Sly Marbo of the Imperial Guard. A renowned Catachan Jungle Fighter who works as a lone operative, and through his legendary skill in stealth, sabotage and guerrilla warfare has taken on entire enemy forces and emerged victorious. In one of his most famous exploits, he took out an entire Ork Gargant single-handedly. In case you haven't clued in by now, he's a mix of Rambo and Solid Snake.
One of the supplement books for Werewolf: The Forsaken, Skinchangers, has a character in it called Shuichi Kurama who is serving as a Host to a fox-spirit named Yoko. The manga/anime series YuYu Hakusho has a fox demon named Yoko Kurama who was mortally wounded and took refuge in the womb of a pregnant woman, Shiori Minamino, and was reborn as her son, Shuichi Minamino. On top of that, the physical descriptions and personalities for both Shuichi Kurama and Yoko Kurama are very, very similar.