First of all I think they should get rid of "power sources". It's really too much of a gamist concept that at first look seems to have interesting potential, but after a while it forces the designers to feel gaps that don't need to be filled. Just identify strong character archetypes and make one class each, then the "source" of its power can and should be explained ad-hoc.
I say just forget about feeble concepts and focus on designing good traditional classes. Something like the Druid is definitely needed in the first PHB, while a Shaman is good for either an early supplement if they can make it fit a large enough array of concepts, from native american inspired shaman to oriental inspired shaman to african inspired shaman etc, or failing that then make multiple versions of it in different suitable campaign settings, but stuff like Warden and Seeker should really get the least priority.
Actually the concept of power sources existed more for story purposes than anything else.
The idea of the primal power source came from the developers asking the questions:
"What is nature magic? Why isn't the druid just a nature cleric?"
The answer the got and the story created around those questions resulted in the primal power source.
The reason the druid was split into the warden, shaman and druid classes is that the classic druid is a rather unfocused class that had a lot of different ablities (combat, spellcasting, shapechange, pet companion, animal summoning).
Sure they were there for a reason, but the designers tried to give the class more focus.
Now, really they should have been subclasses of the druid that shared some abilities (as the runepriest and invoker should have been cleric subclasses) but they only latched onto the idea of subclasses much later. mores the pity.