WORK IN PROGRESS!
While Rock Band is by its own nature not always the most accessible game, there are certain things to keep in mind to help out.
You should make sure your song has properly authored reductions, that are fun to play for people at the skill level that best fits each difficulty.
A big problem with auto-generating reductions or just authoring them wrong is that you can end up with things that can make your chart too difficult to the point of making the game unplayable for some. For example, if someone with chronic pain is not able to strum that much and picks Medium or Easy, you'll want to make sure they won't suddenly have to alt-strum or play fast parts that could give them actual severe pain.
On a wider level, this also helps newer players enjoy and get better at the game, making it so more people can enjoy the game.
You'll want to avoid making your venue too flashy, or it can cause issues for people with epilepsy, migraines and other disabilities, and in a worst case scenario could actually kill someone.
Here are some general pointers on this: