Actually suggested by Elton Williams over at revitforum.org, but a good one.
When building a family with nested families in it and adding a Family Type parameter to it, it works great inside the family editor as it shows just the nested families that are already in the family, but when loaded in your project you can choose each family of that category in your project with the parameter pull-down.
This of course is great for some families, but I would like to at least have some option to just have the nested families that are IN the family to be choosable (I know, not a word) in the project. This will make a lot of things much easier and faster for users as there is much less chance of people breaking families or even freezing Revit when choosing the wrong family in the project.
So maybe a Yes/No parameter in the family, like the one we have for 'Always Vertical'. Something like 'Use nested families in family only'