WikiTimeScale
I have seen many articles about possible timeline projects on wikipedia. I am myself very interested in those and know about EasyTimeline with which you can create timelines in wikipedia articles. Still, that seems pretty complicated and isn't a real solution for visualizing how different wikipedia entries relate to each other in history.
There are however some wikis already in the wild which try to do so. The two most promising I have found are WikiTimeline and WikiTimeScale with WikiTimeScale beeing the bigger one and to my impression also better one.
External Links solution[edit]
It is possible to make an easy external link to WikiTimeScale by entering the URL http://www.wikitimescale.org/en/wiki/ [dead link] and adding the Title of the entry at the end of the URL. Of course you will have to figure out what the title of the entry you want to refer to is in WikiTimeScale. So if you want to link to the Greek empire you will have to use the URL http://www.wikitimescale.org/en/wiki/Greek_Empire .[dead link] Got it? The /en/ in the URL refers to the english version of the entry. To my knowledge, a similar feature in WikiTimeline does not yet exist.
So, all I'm saying is, instead of talking about maybe complex timeline projects in Wikipedia itself, which seem to be never realized, because they are already proposed since month and years without being recognized, it would be a possibility to enter Wikipedia articles into WikiTimeScale and add an external link (see example above) to the Wikipedia article refering to the corresponding entry in WikiTimeScale.
Wikimedia Version solution[edit]
Since WikiTimeScale itself is open source, free for download and kind of easy to install, it is of course also possible for the Wikimedia Foundation to set up this wiki on its own and port it to its needs and style. This would be the more complex solution.