The DMG will have guidelines for creating your own classes and sub classes, as well as new domains such as the death domain. So this space is reserved for when the DMG is released. Until then, by the rules you can't mix sub classes (domains)
The rules for a cleric is that you may only use one domain:
As a cleric, you choose one aspect of your deity’s portfolio to emphasize,and you are granted powers related to that domain.
The rules for multiclassing is that you must choose a class that is not the class you currently have, or an additional level in one of your classes each time you gain a level.
With this rule, you have the option of gaining a level in a new class whenever you advance in level, instead of gaining a level in your current class.
This means that with the current rules, at no time do you have the option of picking up a new subclass or a new domain. But with the DMG other solutions will become available. The Players handbook mentions that there are 12 classes, and does not list any of the class variants or the choices within a class as a new class.
The DMG also has a lot of utilities in it, like for dungeon creation, adventure creation, creating monsters, creating spells, even if you wanted to create a character class. It's not quite the point-buy system from 2nd Edition, but it does say things like "Well if you want to create a class for your campaign then here's a good way to approach it.
From The Escapist Magazine interview here