Well, this is gonna be long.
Having played Vanilla’s questing content quite recently, and the Cata stuff involving the Forsaken before that, the identity of the Forsaken comes down to three main things.
Bloodthirsty vengeance against those who have wronged them, unyielding tenacity and unification in the face of both overwhelming odds and a world that hates and reviles them, and an unapologetic willingness to do anything and everything that would help them secure their continued existence as a people and slaughter those who would get in the way of that existence. That’s what appealed to me when I started playing back in Vanilla and it’s what appeals to me now.
The Forsaken see what they want, figure out what they need to do to get it, and then they do it. It isn’t always successful, but what matters is that no one is whining about honor or complaining and moaning about having to fight their enemies while they do it. The good undead who didn’t like this left, and they’re gone. We see this in the undead who joined the Argent Dawn/Crusade. The Forsaken, from their own point of view and reinforced by their interactions with the other races in the Horde, can only truly rely on themselves, so they hunker down, lock elbows with each other, and do what they perceive as necessary without a care for the opinions of those who care nothing for them as a people.
Some people would point to Lordaeron as the main Forsaken identity, but over my years of playing the race Lordaeron has only ever really been window dressing to me, extra decor for the ruthless personalities that truly make up the Forsaken. It’s also why I roll my eyes whenever someone tries to say Nathanos represents them better than Sylvanas, despite Sylvanas basically embodying all the most important traits of the Forsaken to their fullest while also being the most prolific and important of Arthas’ victims.
The Alliance can reclaim Gilneas and other parts of Lordaeron without completely trashing the race and killing off the character that both leads them and forms the foundation for everything the Forsaken have ever been.