Hoi,
The one major issue I have is that the thinking has been Wikipedia centred. It is in my opinion a major flaw. To top this off, Wikidata is used in other projects as well. When Commons will be wikidatified, it will need article placeholder to bring information in any and all languages for any picture. A similar thing can be said for Wikispecies.
Many items are not represented in any Wikipedia. An increasing number of items are created as placeholders. Many of them could be red links but in many Wikipedias they do not like red links so they are often not created as such. Obviously it is outside the scope of your research but the use of Wikidata extends to all links. It is important to do so because it is a great way to ensure that links actually go where they are supposed to go. It will also ensure that the "concept cloud" as implemented in the Reasonator becomes even more relevant.
What I miss is attention for labels. It is the big elephant in the room. Without labels, the whole placeholder idea falls flat. Search falls flat. In Reasonator it is possible to add missing labels. Once a label has been added, it has an effect on all items where that item is referenced. In a small experiment in Berlin half an hour we was spend adding labels for "Nelson Mandela" and it was gratifying to show how it translated to other people like "president" to "Barack Obama".
Notability in the context of an article placeholder is problematic. If you propose that it has to be decided to have an item as an article placeholder, then I sincerely hope that this is done with a flag. This restricted approach backfires when you consider the small Wikipedias. Arguably it serves a purpose to have article placeholders for any item and particularly when there are labels present. Our mission is to share in the sum of all knowledge after all and this approach to notability has a undesired impact.
Coming back to article placeholder being Wikipedia centric. Is there any reason why article placeholder should not be available on Wikidata itself? Reasonator is not linked to Wikidata at all. It works in any language and, it is my primary tool of maintaining and understanding information in Wikidata. It would be good when Reasonator is replaced by something native and article placeholder is the obvious candidate.
I am happy that article placeholder is happening. It will have an interesting future.
Thanks,
GerardM