|Early days of our sandbox hex crawl|
In a sandbox campaign, the world is in motion. Bad guys have plots. Good guys have plots. Good guys have bad plots. Heck, even bad guys can become allies. In my last campaign, every region had a complex balance of power. If you go kill the dragon, who was the dragon keeping from realizing their plot? Is it someone worse? You better find out.
With the sandbox, the players may come back to find the town has changed dramatically. Plots have come to fruition. Perhaps bad guys have exacted their revenge or good guys finally got what they wanted. This is not unlike real life, and it's not failure. It's just some things that happened. The characters may shrug or they may decide to work against change. There's no wrong answer.
Everyone has a plan and a timeline in the sandbox campaign. The PCs may destroy a group, slow a group, or accelerate the plans of a group. In my last campaign, the PCs destroyed the evil satyrs that burned their town down each year. They took the battle to them, along with their allies, and wiped them out. That threat was no more. Meanwhile, it took them until around 12th level to take the fight to the evil Cyclops necromancer. During that nine months of game time, two "undead apocalypse" scenarios unfolded in the region, leading to death and destruction as the party tried to figure out what was going on. They were busy elsewhere at the time. When it came to the Big Bad Guy, they never stopped him, but they slowed down his plans, pushing his schemes outside the window of the campaign (now we begin the second campaign).
Besides schemes and timelines, in the sandbox, there are repercussions to actions that we don't see in a standard campaign. If you annoy the mayor, expect long term trouble getting people to work with you. If you raid the rakshasa's lair and don't kill him (as happened in the first campaign), he will most definitely be screwing with you. The world is in motion and people are not dungeon mooks. They have friends and allies and resources to bring to bear on those who hurt them. This might also mean that a group of mercenaries who mercilessly kill and silence their enemies while rising in power, might be seen as a threat. Politics and social interactions go a long way in keeping a positive narrative. Suddenly the bard is incredibly useful.
What's lacking in the sandbox is usually a long path of adventuring that precludes understanding these various interactions. If your characters just spent five levels of adventuring out in the hills in a dungeon, do they really care what's happening in town? Their life is in that dungeon. That means adventures, as normally understood, are short. Half a dozen encounters is a long adventure in the sandbox. Mega dungeons will derail a sandbox. A disappearance of even a few weeks means a lot can change back in town, a strategy a bad guy should consider. I don't need to kill you, how about I send you on a wild goose chase or trap you on a time dilated plane (the rakshasa again). Being sidelined in a world in motion is a terrible thing, as all the work to understand various factions and schemes becomes entirely irrelevant. It's like leaving your Sim City computer game on auto pilot over the weekend. In a standard campaign, plot and schemes usually just point to the location of the next hobo murder.