@DialogFactory works great if you know exactly how many tabs you are going to set in you dialog, but in @Template you have the little extra that you can define everything (even the number of tabs, that are dynamically created) in the method annotated with @DialogFactory.
There should be a way to do the same even with classes annotated with the @DialogFactory, outside @Template. Hypothesis:

@DialogFactories
public class MyDialogFactories {
@DialogFactory
public void bannerDialog(...) {}
}

IMHO the cleanest way should be to have an annotation at class level (which works for paragraphs and as standalone) and an annotation at level method for initialization (which works on templates, paragraphs and classes annotated with the other annotation).