I think, at some point, it is better to have an actual Core, and a series of additions that can be added to the Core. If everything is Core, then one presumably has to master everything to run the game. As the game evolves, the amount of Core material becomes staggering, and people who don't have the time to read thousands of pages of material are going to find something else to play.
The philosophy I am describing -- that which was foundational to previous editions -- is that flavour defines setting, and the mechanics are intended to support the flavour. I.e., setting-first design.
In the case of Runecasting, I did use that in non-Norse settings. I also used several other setting-specific rules in order to craft my own setting. And I would agree that it would be useful to have compendiums of options, which a prospective GM can use to craft a setting....or to inspire his/her own house rules to craft a setting.
"Everything is Core" seems, to me, to exist only to ensure that most players will either get a DDI subscription, or will buy (almost) everything. Options are things you don't necessarily need to buy. This might be a good business decision (if players buy into it, it is a good business decision), but it is a poor game design decision IMHO.
Frankly, not every setting needs rules for casting spells through djinn, for three moons of magic, or for Gothic horror.
RC