So, something like AD&D Historical Reference Earth with the following modifications:
1. Elves, dwarves, goblins, fantastical monsters and so on are natives of Toril, and found on Earth only if they or their ancestors wandered through a portal. So, for example, Grendel might have been born on Earth, but his mother was from Toril.
2. Magic is possible on Earth, but most spellcasting traditions derive from contact with natives of Toril.
3. All tales of Faerie and other fantastic magical locations (though not Heaven and Hell, Asgard, Olympus, etc., which are the same Outer Planes familiar to D&D lore) actually refer to portals to the Forgotten Realms. So, for example, the magical lake where Sir Lancelot was raised was a portal to Toril. Nimue was a queen of a nation in Faerun. The Forgotten Realms can still have a Faerie/Feywild of its own...
4. Many of the human cultures in Faerun are descended from human migrants from Earth, some of whom brought their religions with them.
5. Earth has its own gods and pantheons, some of which spread to Toril with human migrations. Toril has native gods of its own, though.
From the 1st edition Forgotten Realms Campaign Set: "The world of the Forgotten Realms, Abeir-Toril, has a huge number of portals and other gate- type devices which link that world with other areas of the Realms, with the Outer Planes, and with alternative material planes containing other AD&D campaigns...
"Characters making the transfer, by whatever means, to the Realms gain the ability to speak the Common tongue of the Realms, as well as the Realms equivalents of Elvish, Dwarvish, and other languages...
"Similarly, magical items that are "special" to a particular world may not make the transfer to the Realms, either being destroyed, being returned to their original plane, or being stripped of the player and placed somewhere in the Realms...
"The physics of the Realms are slightly out of sync with the rest of the planes, so that gunpowder and many technological devices which operate on electronics do not function...."
Time Gates: From Cormanthyr: Empire of the Elves: "There are time gates on Faerun, but their locations remain a mystery. Ancient texts of Mystryl's faith talk of three gates that were created either through accidents or great fortune. Unknown to any other races on Toril, there is one other time gate, and that is known only to the High Mages of the elves. The time gates are older constructs than anything else in the Realms, either Arcane Age or modern time periods. They predate the first dwarven realm on Faerun, and even the elves seem in awe of their age and power."
The time gates should be usable to travel to either Earth or Toril, and those flung to a random time and place might end up on either world.
Netherese portals: Anauroch, page 68: "The Buried Realms are (correctly) said to be riddled with ancient, unmarked magical gates, allowing passage—often unintentional!—from the Realms to other planes and worlds, and vice versa." This source mentions portals leading to Ravenloft and Krynn.
Imaskari portals: in Egypt, Mesopotamia
Panchengjuduohuh, the Sunken City of Many Doors, from OA5 Mad Monkey vs. Dragon Claw: "While it looks ruined, the City of Endless Doors is really a mazework with doors leading into various areas of the Eastern Realms, the Forgotten Realms, alternate material planes, and upper and lower planes."
The Cave of the Monkey, from The Horde boxed set: "The cave is more extensive than just these apartments, however. Elsewhere are gates to other Prime Material planes, other worlds on this Prime Material plane, and other planar locations... Always the trickster, Monkey has made his gates special. Those that open on the Prime Material plane can be crossed easily from the outside. To leave, however, requires a special code phrase. People who come in often find it very difficult to leave..."
Cloak Wood: "The sages of Candlekeep have sufficient evidence to indicate at least one gate exits in the wood, but the exact numbers and/or destination of these gates is unknown. They may lead to other parts of the Realms, to an Alternate Material Plane where such creatures are common, or to the Beastlands (Happy Hunting Grounds)."
Cormanthyr: Empire of the Elves details the Elven High Magic rituals needed to create portals.
The Myth Drannor gates, in particular, are likely to tie to Earth.Ghaatiil/The Traveling Path: After two High Mages individually perform a vuorl'kyshuf ritual to allow communication across great distances, a third caster maintains this link during the ritual. All three unite in this Complement ritual to link the two sites with a gate, setting its operating conditions during the ritual. Visually, each High Mage is surrounded by a light mist; as each performs the rite, he is joined by a ghostly apparition of the counterpart High Mage at the other site. The ritual ends when each High Mage uses the gate to join the other to close out the ceremony at first one and then the other gate site.
This establishes a permanent gate at the sites of the ritual. Historically, High Mages created these at the behest of their Coronals as well as their own wishes. Their express purpose was to allow travel among the elven capital cities and strategically important strongholds. The ritual established many strictures over its gate's use by type of elf (race, occupation, or status) but often set the portal to activate only in the presence of a key (anything from a physical key to a lock of golden hair or the sound of a whistled tune). However, Myth Drannor's High Mages established an increasing number of open gates to allow greater access and alliances with far-away lands and peoples. While a High Mage (and some wizards) can easily close down a gate, the existence of so many worries some elves, despite the assuaging words of the Coronal Eltargrim.
One-way gates can be created by this ritual, though this becomes a ritual for four High Mages. One High Mage performs the standard gate construction, while the other three mentally corroborate each others' knowledge of the gate's exit point. The exit must be a place physically visited by at least three of the High Mages in the ritual to allow success.
Asram has (had?) a portal to Finland, which the goddess Kiputytto entered to take on Talona.
Sschindrylryn's Gate: From Drizzt Do'Urden's Guide to the Underdark, page 68. An ancient gate near the drow city of Menzoberranzan; the drow there have no idea where it leads. To open it requires drinking dragon's blood and casting an alteration spell.
WotC had a series of articles on its website called Perilous Gateways, exploring the various portals of the Forgotten Realms. These portals are all assumed to link the Realms with other places on that world (or, occasionally, places like the Ethereal Plane) but in a campaign that made more of the connections between the Realms and Earth, many of them could easily be on Earth instead. For example:
Delfondar's Map leads to a portal not in the Giantspire Mountains, but to a portal somewhere on Earth, and only afterwards leads to the Giantspires and beyond.
The Moonstars are based somewhere on Earth, with portals leading to various cities in Faerun. After Khelben "Blackstaff" Arunsun's exile from the Harpers, he moved to Earth but continues to use portals to keep track of events in the Realms.
The Raiding Portals were used by a lost civilization to connect to farming regions on Earth; bandits in the High Forest now use it to raid Earth's countryside.
The Escape Portal leads to Earth too.
The Isle of Prespur is actually the Isle of Prospero on Earth; he's still a Netherese mage called Prespur, but he was known as Prospero when he was Duke of Milan.
The Temple in the Forbidden Jungle is in Central America, not Chult.
The Dusty Rat Portal in Amn leads to Earth, and is a source of confused, naked adventurers from Toril. It could still lead to a vampire tower there.
Rikkine's portal is supposedly the second portal ever made, so it probably leads somewhere significant. Atlantis?
Bane's Eye. Change "three places around Faerûn" to "three places around Earth."