My primary exposure to D&D prior to playing the game has been watching Penny Arcade's annual or semiannual Acquisitions Incorporated games. While these are very fun (largely due to fun characters and Chris Perkins' ever-game DM'ing,) they are generally on rails. There's very little real "dungeon crawling" and mostly involves single-sitting adventures that typically only involve one large fight rather than multiple smaller ones.
It makes things very entertaining and works to give each two-and-a-half-hour session a clear arc. But as someone who doesn't need to entertain a crowd, I've been exploring a more open-ended adventure.
I spent almost a month putting together an adventure that takes my players to a location they have never heard of (though they've encountered some of its inhabitants.) I'm going to be slightly cagey about this on the off chance that any of them follow this blog, but I'll talk a bit about structure.
The idea behind this is that there is a bordered region that's going to really be where they do their adventure - if the players attempt to leave this area, I might have them encounter extra-tough enemies or simply improvise some new settings for them to visit. But generally, I'll try to steer them back into the adventure region.
However, while they will be somewhat limited to this area for what I imagine will be many sessions, while they're in there, I'm going to try to be pretty open to what they want to do and where they want to go.
My strategy for preparing this adventure was to first come up with a general sense of the "overworld" area. This is a large region that will require days of travel and special threats that they'll deal with the entire time.
My expectation is that the players will level up at least two times (probably more) by the end of the adventure. The overworld region won't level up with them, so threats that seemed pretty scary when they first got there will, hopefully, seem like something they can overcome with ease.
One way to help dull the difficulty when they first arrive is that they'll get a couple NPC party members. Eventually, they'll be able to get three followers who will always be two levels below them. These guys will dilute the XP they earn (I don't want them getting super-powered before they escape) but also make combat encounters less likely to wipe them out.
From the Overworld, which is totally set up to cater to their current level, they'll go to various important locations, which is where most of the plot of the adventure takes place. These areas are "tuned" for higher levels, with more challenging beings that rule over them. My thought is that each of these regions will take maybe one to three sessions, and while one of them is the "final dungeon," they're designed to be relatively self-contained, so that the players can do them in any order.
My thought is that, especially with noncombat alternatives to some of the key conflicts, it won't be a problem if the players outlevel a region. The environments and characters are, I hope, enough to make easy combat still entertaining. On the other hand, I'll be curious to see if there will be situations where the players feel compelled to retreat from combat.
It's a big experiment, but I'm really excited to see how it works out, as this kind of open-world design could be a model going forward. (It also means that I probably won't have to prepare any new content for months.)