Scene
The campaign will mainly take place in the Khomand League of Principalities. Khomand is a group of eight loosely affiliated city-states, each ruled over by its own prince. As signatories of the Great Compact, they are bound to defend each other from attack and allow (mostly) open commerce between them. The lands are mainly temperate and inviting, with fertile farmland and woods in the south; sweeping, open plains in the center; and rugged hills and mountains in the north.
The campaign will also cross over into the Kingdom of Aedra, although this kingdom will mainly play a supporting role. Aedra is a fully united monarchy, replete with fertile lands, woods, and a rather Mediterranean climate. Aedra has recently crowned a new king, and is beginning to open up to greater interaction and exchange with her neighbors.
The player will likely cross into the Orcish Tribal Lands during their adventures. These lands are generally very similar to Khomand, but seem to be patrolled by roving bands of orcs who steal and destroy rather than cultivate crops. In truth, there are several permanent orc settlements, and even a grand capitol city, but no outsider has ever seen them and returned to tell of it. the southern and northern reaches of their lands are covered in huge forests, with fertile coastal plains. The borderlands near Khomand are largely wilderness, left fallow.
Finally, the players may cross into the Wastes of Eglath, where the hotter temperatures and lack of cultivation have created a wild and unforgiving savannah. There are some fertile stretches along the riverbanks, and more imperial age ruins can be found in Eglath than anywhere else.
People
Khomand and Aedra are human nations, but they include minority populations of dwarves, gnomes, and (rarely) elves and half-elves. Half-orcs, while not unknown, are exceedingly rare, and viewed with suspicion and fear. In the largest cities, you may even encounter a dragonborn or two. Although the nations are ruled by human nobles, all races are (supposed to be) equal in the eyes of the law. Active in the human nations are four key organizations: the Guardians of the White Flame, the Ravensbrand, the Brotherhood of Arcane Studies, and the Merchants' Guild.
The Guardians are a powerful knightly order that is mostly concerned with maintaining the current social order in a peaceful world. They will require the player to join their order to reach key story beats. There are several factions within the order which will vie for the player's support. There is also one key antagonist in the order, who wishes to supplant the player and gain greater influence himself.
The Ravensbrand is a thieves' guild with influence across much of Khomand and Aedra. They will take an interest in the player, and wish to use them to further enrich themselves. Interestingly, the Ravensbrand hold to a unique code of honor, and may become key allies if the player develops this relationship.
The Brotherhood of Arcane Studies, or simply Arcanists, prize magical knowledge above all else. However ill-advised some of their research may be, life would be far worse in Aedratha without the simple magical tools that they have developed. Their only interest in the player comes from any rare magics or magical artifacts the player might be able to deliver to them, or (if wealthy enough) patronage of their continuing research. Largely disorganized and individualistic, the Arcanists can still shift the tides of battle through sheer magical power.
The Merchant's Guild is predictably concerned with amassing wealth, although Guild membership does impose certain standards and requirements on the members. The Guild never forgets a favor owed them, and rarely owes favors themselves. The Guild will likely hire the player for some key jobs throughout the story, and may provide significant support in the future, should they be impressed.
Should the player encounter orcs, it will likely be in combat. If by any chance the player does interact with orcs outside of combat, the orcs mainly wish to protect their families from human expansion and bring honor to their tribe. Pirates have been raiding the southern tribes and taking slaves. The orcs don't see a difference between human nations, and so have been retaliating against Khomand. They are a surprisingly peaceful, and honorable, race of warriors.
The gnoll packs, goblins, and other inhabitants of Eglath that the player will encounter are enemy combatants who have given command to demons. Azanoth, the demon leader, wishes to make his captured town into a fortress from which increasing waves of demons will be able to launch their campaigns of destruction. He also wishes to torture the captured inhabitants of the town for years to come, both to revel in their suffering and to siphon that energy to increase his own strength.
Best of luck writing your D&D duet this worldember! It's a fun goal, and it sounds like you've got a good idea of what you want to write!