While nearly all types of game content are welcome in the Dungeon Masters Guild, there are certain types we think are more in demand, and which we will tend to promote more prominently on the DMsGuild marketplace. Primarily, we’d like to see short adventures that can be run in a single session, especially ones that can complement larger campaigns already published by Wizards (e.g., Rage of Demons, Curse of Strahd). Also in demand are 5th Edition conversions of monsters absent from the 5th Edition Monster Manual and new backgrounds for players to use with their characters. If you're inclined to make visual media, maps and character art bundles are also welcome to enhance campaigns. Again, we will tend to emphasize content that would be useful for other D&D players looking to expand on existing official storylines, such as a collection of vampire spawn or other lesser undead useful for Curse of Strahd.
At the end of the day, though, the best content for you to write is the content you’re inspired to create. The Realms is a big place. Ravenloft is a scary place. We look forward to seeing what you create for them!