I have SimCity Complete Edition. And there is at least one DLC that is Locked and unable to be accessed unless you buy The Sims 2 on Origin, which last time I checked, has been unavailable for purchase for quite some time.
I was really, really, disappointed when I discovered the amount of DLC content missing from the "Complete" version of the game. EA has totally mis-labeled the product. a more accurate title would be "SimCity Incomplete Edition"
As for the sponsored DLC, I.E. Crest DLC, Progressive DLC, etc., It should not be that hard for EA to create generic versions of the DLC that they could provide to the SimCity Masses (us users) either for free, or for a small fee. If they believe they can't because they (foolishly) let the dev team go, then there is a vast and wide community of SimCity content creators that they could contract to make the items. The only thing really stopping this kind of thing is EA's stupidity (but then, we all know that they are known for that.)
I have been waiting and waiting for someone from the community that would create the generic form of the DLC not freely available that could be freely added to the game, but so far, I have not found it.
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As for the country specific DLC, I have managed to get all of those, but it was only due to an enourmous effort by me, and it was an enourmous pain to get them. I don't remember all of what I did, but I remember it involved using tunnelbear to connect to origin in the countries that would allow the DLC to be accessed. If anybodies interested, I could try and find the directions I followed and post a link to them.