Creating an adventure or campaign setting for a Dungeons & Dragons game is a daunting task for any Dungeon Master. Thankfully, Wizards of the Coast has put out a plethora of pre-written adventures and fully fleshed-out fantasy worlds to set them in. Newer DMs looking to step up their game and surprise players don't need to jump right to making everything from scratch -- there are plenty of ways to modify existing adventures to suit any group's play style.
A DMs first and best resource for all forms of campaign modification is the Dungeon Masters Guild, a free to join site that is home to a repository of supplements, both officially released and fan-made. In fact, there are some supplements for use in official adventures on the DMs Guild that are written and designed by the very people who worked on that game.
It is a storefront, so many of the modules cost a small amount to purchase. However, there are a lot of helpful supplements available to download for free that DMs. Using the DMs Guild is simply the easiest and least time-consuming way to approach implementing the following tips.
D&D Adventurers League Modules
The D&D Adventurers League is Wizards' officially coordinated living campaign, where anyone can register a character and hop in a game. Modules are released that bit-by-bit build up their own story, but every season of the Adventurers League is tied to an officially released adventure.
So, if a DM is getting ready to run Tomb of Annihilation but they want to add more, they can use add-ons designed by Wizards specifically for that game. This is a great option for DMs who are wary of fan-made supplements being unbalanced. It's a simple way to spice up your campaign and get players more invested.
Fan-Made Adventures
For those willing to get more experimental, there are tons of fan-made adventures that can be plugged into a pre-written adventure. Some offer things like alternative introductory stories, and others are side-quests designed to be inserted mid-campaign. Many are made with a specific adventure in mind, but most of these supplements, with a little work, fit into nearly any game. This is a great option for DMs dealing with players who are already familiar with the official adventures. Nothing is better for a D&D game than the unexpected.
Monster, Trap and Magic Item Supplements
Maybe a DM isn't looking to add more story, so much as modify aspects of it. Traps can be made harder, familiar monsters can be replaced with an unknown foe the players can't meta their way through and magical items (cursed or otherwise) can be sprinkled in as a DM sees fit.
Just like the custom adventures, many of these supplements are written with a specific campaign in mind. Going back to Tomb of Annihilation, there are supplements that add new monsters, items, and encounters both in and out of the jungle. There are also more generic supplements that may still have that little extra something needed to elevate the experience.
Incorporate the Player Characters' Backstories
One of Fifth Edition best additions is backgrounds that must be chosen when creating characters. These provide context to who this player character was before becoming an adventurer. 5e also requires players to come up with personality traits and motivations, meaning every character ends up being more fully realized than a bunch of simple numbers on a page.
Once these choices are made, a multitude of new threads can appear. Is the Paladin on a quest for their patron god to find a holy relic? Maybe the Wizard is seeking revenge for whoever slew their teacher. The point is that instead of having to come up with side plots from scratch, a DM can find inspiration in their players. When the stakes personal, the party will likely be more engaged in the story as a whole.
Add House Rules
Finally, there is the long-honored tradition of house rules. D&D is a framework for cooperative storytelling, but it isn't set in stone. If there is a part of the rules that a DM feels is a hindrance, they can always change it. The players must be informed and agree to the changes, but oftentimes, simply tweaking a few rules is enough to drastically change the way an adventure feels.
For example, if DMs are looking to increase the deadliness of combat, they can rule that critical hits always deal maximum damage. If the idea is to make the players feel more heroic, a DM can increase the amount of inspiration awarded beyond one. This can lead to a more sensational and forgiving game. There's no end to how a DM can tinker with the rules as long as the players are on board.