I've noticed that as the game has gone on we've begun to deal with much darker themes.
Core Game: You lose someone close to you. It began with your mentor, someone you trusted and cared for who sacrificed themselves so that you could push onward to protect others. Next it was a partner, it might've been an optimistic Demolitionist or a resolute Lionguard who stood with you against the tide. Or it might be someone you thought you could trust who turned to plant a dagger in your back. These hurt, they were painful experiences that sometimes you can't save everyone and that other people believe or fear you enough to take matters into their own hands regardless of consequence.
S1: The world is shaken by a madman. You see enemies thought subjugated rise up to stir up troubles just as you've gained a foothold against the Elder Dragons, upsetting what you've striven for so you once more rally allies and take to the front lines to protect others. In the final culmination, you ultimately lose because while the madman is defeated you find they have completed their mission with a grin on their face.
S2: Your allies who helped you become the Commander begin losing their loved ones. New friends find themselves torn apart from within. Old comrades betray you and refuse to explain themselves. Everything begins to shake apart as no one has a cohesive goal anymore. So you bind them all together again facing another Elder Dragon--another threat to make the world focus.
HoT: You lose almost all of your allies in a single war. The Pact is a shadow of its former self. The world no longer has any faith in it except as a ragtag collection of the Three Orders. They've lost their Marshall. Nearly all of the heroes from Destiny's Edge are damaged in some way, your own new allies are injured, battered, and bruised, and while you ultimately come out triumphant with a new hope born in Tarir the world looks at the Pact as a bunch of dreamers who nearly brought their own ruin. Only by dint of your own status do you remain anything of note in this new world.
S3: Ancient enemies resurge. New chaos befalls the various races of the world. Ancient magics return and a Rogue God's pride begins a spiral into damnation. You chase these threats across the Tyria trying to put out a hundred small fires from every corner while your own allies disappear with their own agendas. It's a race to see what you can accomplish when the clock starts ticking against you, and when you finally catch up it slips away to pursue an even more reckless backup plan and you no longer have your allies with you.
PoF: Faith, resentment, exhaustion, illness, beseige your old friends. The Commander barely has any allies left and so you start hunting for new ones, trying to build up a force to stand against this threat. And while you're busy trying to keep this war from destroying the world the larger portion of the world completely ignores the problems or stays out and watches from a safe distance while you and those few who are strong enough to still stand with you struggle against the hurricane. Although you succeed, you find this has simply shifted the problem from one threat to another.
S4: A time of despair. Friends suffer. Friends die. Friends begin to pull away and pursue their own lives, looking for happiness in a world that can never seem to be at peace. Even though some reconcile with you, others can't continue the fight and step aside or sacrifice themselves. And your enemies can all see it--they see that no matter what you do the world keeps slipping into darkness even though you've brought a new Light to the world. And when the dust settles, the beast slain, and the dice stop rolling, every single person who has joined you throughout this now questions if YOU are the biggest threat there is.
IS Prologue: That fear of you begins a tangible thing, and you find yourself begin confronted on it. Even though they sparr around you with your allies, the Commander is being seen a less of a hero and more of a threat to be cautiously evaluated.
IS: The Commander and his allies are now a threat. Something to be eliminated. While you have allies who stand with you, you begin to grate against them. Darker choices are made, mercy becomes a luxury. And all the while a threat is stirring and has taken a personal interest in you with the hope you could become an ally.
The game has gone from the hopeful rise of a hero to the desparate struggles of someone to save a world that is selfish, paranoid, and cruel.
And I love it!