As most here might ( or might not ) know, the green skins are neither one race nor a unified nations. Green skins simply summarizes a multitude of clans, tribes and subspecies; some more dominant in the lore, or fleshed out in the Tabletop than others.
As someone who collects a green skin TT-army himself, one of the biggest strengths of the Orcs and Goblins army book is the great selection of thematic armies it allows you to build.
You see, while the Empire army book allows you to chose between like six very different Lord and Hero options ( Captains, Witch Hunters, Warrior Priests, Wizards etc ) the Orcs & Goblins one only offeres you two: Bosses and Shamans.
BUT
It allows you to pick from a good range of different green skin subspecies and clans.
So can the Warboss leading your army for example be an Orc, Black Orc, Savage Orc, Night Goblin, Goblin ( or Forest Goblin ).
My tabletop army for example is all Orcs, with the exception a single unit of Black Orcs; while a friend of mine has an army thats almost nothing but Night Goblins but with some Spider Riders mixed in there.
Thats ofcourse completely optional, and there is nothing stopping you from throwing all sorts of green skins into a unified army. Afterall, Orc tribes often enslave weaker Goblin tribes, and a powerful enough Warboss can unite dozens of tribes under his banner.
But as someone who is a fan of "fluff" armies ( army lists build around a certain theme or a specific part of the lore ) as they are often called in TT-circles, I really hope the game is not only going to give us the option to play with themed armies ourselves ( looking positively on that front ) but also allows us to encounter them in form of minor, non playable factions.
To get to my point: I hope not all of the npc-tribes we have to deal with in the game will have access to the full roster of green skin units; and there are at least some that are themed to represent "pure", non mixed societies and are lead by members of their breed or culture.
I think it could be much more interessting when discovering a new tribe means dealing with new army compositions and fighting styles. Like running into your first Savage Orc tribe after having dealt mostly with Goblin tribes so far for example.
Orcs
Savage Orcs
Goblins
Nightgoblins
Black Orcs
Forest Goblins