For one thing, I would use some published modules. When I originally ran it via Play by Post, I used all original material(except for a few standard creatures from OSRIC). PBP is good at that--you have lots of time to plan the next step and just go with the flow. In a live game session, on the other hand, it's harder to give your dungeons the same depth when you're making it up as you go.
So now, I can think of four published modules to include in the sandbox, in addition to original content. I would, of course reskin them, to give them my own touch and to try and obfuscate them a bit in case any of the players have read/played them in the past.
So to begin the campaign I would probably tell the party it's the start of Winter and they are new arrivals in the area, having just arrived by boat in the city of Warren's Deep.They can root around town looking for work or adventure(perhaps in the "Sea Elf's Daughter"). Here are some of the rumors they might hear:
- They say the Mayor occasionally hires mercenaries for tasks he doesn't want associated with himself. They say he likes using outsiders like you. His agent is supposedly the Pawnbroker in the Alley of Felicious Delights, though it wasn't me who told you that.
- The Lord of Dappenshire and many of his knights were recently killed by blood ravens while rescuing his kidnapped son. Since then, his troops have been ineffective against the Orc raiders and his widow has put a bounty on each Orc head.
- Rumor has it that Lord Derivas, whose fiefdom lies just outside the city, is vying to raise his influence in the city and is seeking allies.
- Something is rotten in the village of Wankerly, in Kipperton Fiefdom.
- The Watch has put a bounty on the murderer of a watchman. The problem is, they don't know who he is. Also, people in the Capel Gwyn slums, the scene of the crime, don't like to cooperate with Watchmen.
- Some small-time local heroes made a name for themselves, and quite a bit of money, round here last summer. They were last seen going off into the forest last summer. They haven't been heard from since, but there must be something out there to interest them.
- In the swamps to the South, wrecks from some, long-ago magical war occasionally surface from under the muck. Brave souls occasionally return from there with ancient wonders, but more often they find nothing or don't return at all.
- A merchant ship working the coast is looking to hire some extra protection due to recent Pirate attacks.
- A crazed sailor claims to be the sole survivor of a fishing boat that was lost in a storm. He claims to have discovered an Island that has risen from the bottom of the sea and contains great treasures of the ancients. He'll take you there if you hire a boat and give him an equal share of the treasure. He gets a wistful, doomed look in his eye that you're not sure you like.
- There is an old ruined fortress on a small island just off the coast of the seaside village of Smarmytown. Rumor has it that a humongous pearl lies there, but that none who have sought it have ever returned.
Some of these are from my original campaign, some of them are new, and some actually refer to events that happened in that game. Rumor 6, for instance, sets this campaign as being a few months after that campaign. I realize this continuity doesn't interest anybody but myself, but that's the thing about world building--if you want to do it right, you should probably just embrace the fact that it's 90% for you and 10% for the players who only really get to spent a very short time in your world.