There is no single ideal way to handle DLC, as every project has different requirements and goals. Still, here’s some general information on handling DLC that should be useful:
All DLC projects must use the same GUID for their master bus as your game project does, to ensure that all content eventually routes into that bus. In addition, if you want your players to be able to route their events into your projects’ buses, those buses (with the same GUIDs) will have to be included in their projects as well. If you want them to be able to create new buses of their own, you’ll need your players to build their master banks as well as their content banks so that the new buses they create can be loaded in your game.
Often easiest way to achieve this is to make a copy of your project, strip out all the events, and then give that skeletal project to your players for them to create new events in.
Once you have banks created from multiple different versions of your project, all you need to do is load them at the same time as your existing ones in order to use content from multiple banks.
When loading multiple versions of the same bus or event due to that content being present in more than one loaded bank, the version that was loaded first will be used.
Incidentally, there is no limit to the number of banks from the same project that may be simultaneously in memory, besides that imposed by the available resources of your target platform.