Ah but you see, now because in 5e domains = cleric subclass, you're stuck with the idea that they have to be as broad as other classes subclasses. They can be broad if you want, but they don't have to. If this is the result then perhaps it was a mistake to make domains into subclasses.
I don't think there was neither any conceptual nor practical problem in 3e with having 50 or so different domains. Furthermore, in 5e domains still grant a list of domain spells plus a few bonus abilities (with many overlapping of abilities across different domains), so it is definitely a piece of cake to design hundreds of new domains just by swapping some domain spells with other spells.
The downside of broad domains, is that there is a risk of sometimes not really capturing the idea of a clerical faith (see the Death domain as the biggest example so far, it is not really usable for "good" religions of the dead). If you don't want to "officially" design a new domain, another option is to allow a specific character (or some NPC) to swap domain spells with others.
). If you want an anti-dragon abilities, go for the Nomling domain.
