Almost all fantasy campaigns begin with natives of the fantasy world operating in their own world. An alternative that is seldom explored in RPGs is having characters from our world enter the fantasy world. There are many examples of this in literature, including The Chronicles of Narnia, Guardians of the Flame, The Chronicles of Thomas Covenant, The Spearwielder’s Tale, Three Hearts and Three Lions, Mordant’s Need, Glory Road, A Wizard in Rhyme, the Spellsong Cycle, the Barsoom series, Magic Kingdom of Landover, the Dragon Knight series, the Gandalara Cycle, the Darwath series, the Spellsinger series, Bifrost Guardians, and Witch World.
Characters from this world may find a portal to the fantasy world and enter the portal either purposefully or by accident. They may also be drawn into the fantasy world by someone seeking their help — either for good or for evil.
Your players may go as themselves, in which case you and they will need to quantify their ability scores and skills. This may cause some hurt feelings — it is probably better to error on the side of the heroic.
Characters can also be placed into new bodies when they transfer to the fantasy world. This approach lets players keep their existing Int, Wis, and Cha scores, but gives them new Str, Dex, and Con scores. A variant of this is to merge the characters from our world with existing characters from the fantasy world, where the merged character has the memories and abilities of both source characters.
Another option is to roll up purely fictional characters from our world who will then be relocated to the fantasy world. This allows players take on the roles of skilled fighters or brilliant scientists from our world who are transported into the fantasy world. Perhaps your story begins with an elite special operations unit being summoned across dimensions by an evil wizard…
Depending upon how your character make the transition to the other world, they may or may not have time to prepare and equip themselves. Or, the means of transition may limit how much or what types of equipment they can bring with them. In some campaigns, you may rule that technology from our world simply does not function in the other world.
If characters are allowed to equip themselves for the transfer, they might choose to bring along modern weapons and ammunition. They might also bring along items such as binoculars, lighters, and two-way radios with solar chargers. A smart character might bring along a compound bow, because copies of the advanced bow might be able to be created locally after the transition. The smartest characters will bring along scientific textbooks and equipment such as compasses, clocks, stopwatches, multimeters, calculators, thermometers, microscopes and telescopes.
Unless some of the characters gained arcane or divine spellcasting abilities in the transition, you may need to consider how to work characters gaining those abilities into your story line. Certainly, all characters can start out in the martial classes — particularly if they brought modern reliable firearms and ammunition with them. The conversion of a character to a local religion or the development of arcane powers will almost certainly make a longer campaign more interesting.
If the method of transition to the fantasy world is one-way, characters may be trapped there with no means of resupply. If the characters can move between the two worlds, this opens up possibilities for resupply, trade, and… other users of the portal. Perhaps evil denizens from the fantasy world decide to follow the characters back home, or evil people from our world decide to take over the fantasy world…
The flow of time may effect travel between the two worlds. Perhaps years pass in the fantasy world while only days pass in the mundane world. This will certainly effect resupply or trade, as a simple visit to the mundane world could lead to the character being gone from the fantasy world for six months.
You will also have to decide how items brought back from the fantasy world operate in our mundane world.