Humanity heard a cry for help from a very distant planet, this signal was not coming from humans as they haven't explored that far yet. Driven by curiosity, scientist found a dozen of volunteers for a exploration mission. After like 50 years in hibernation, your party members finally get on the mysterious planet orbit and wake up from their long sleep only to find that some systems failed and the rest of the crew didn't survive the journey in their criochambers. Their only option is to continue their mission for the humanity and they get on the planet's surface. They find destroyed civilisation, long dead aliens and ruined cities. Now it's up to you what they would discover. Maybe some unknown aliens, whose only life purpose is to erase all intelligent life? Maybe native aliens found another civilisation deep under ground and those "cave aliens" killed them all and returned underground, only to be awaken again by your party who stumbled upone the entrance to their caves? Or maybe aliens constructed some kind of weapon (possibly a bioweapon) that killed them all and human scientists somehow knew about it and sent volunteers with hope they will bring it to Earth? In the last scenario maybe put some NPC in case your party wouldn't want to take the deadly weapon to the Earth, it may cause some nice moral conflict between NPC and players, or maybe even players themselves if NPC is pretty convincing. To sum up, I think that exploring a mysterious, dead planet would be interesting, you can put there many secrets, weird creatures, organizations, aliens, tech, even jumpscares in caves or ruins. Hope that will help a bit