This already happened where I grew up. Three separate churches (The Methodists, The Baptists and the URC, there where talks with the Anglicans as well but for various reasons it would be tedious to lay out they pulled out of the project) now use the same building for services, because none of them had enough 'bums on seats' to keep their respective buildings open on their own. Over time, the congregations are slowly merging, with people attending services based on convenience or whether they like a particular preacher/minister rather than on the official denomination of the service.
I believe the (British) Methodists asked to rejoin the Anglican church in the 1970s, but the CoE synod declined.
You are however right that they appear to be moving closer again with shared buildings and even services, and this is accelerating. The CoE is very broad-based and caters for a lot of different views, so it's not hard to imagine the Methodists and URC (I'm not so sure about the Baptists) fitting in okay. On the other hand, I could imagine the internal politics of the CoE blocking a formal reuinion, because several wings might be unwilling to see their influence diluted by the new congregationalists.
To Nitpick on point #1 - there hasn't, from what we can tell, ever been a point in history when all the (major) christian groups had a single canon that they all accepted. We do know that prior to the councils of Nicea there was no agreed upon canon nor was there one resulting from those councils.
Sorry yes that is what I was trying to imply. The Bible became what was "canon", but of course the bible produced was heavily contradictory and therefore, though it was "canon", and methaphorically everyone now 'drew water from the same well', they still produced horiffically different results because the canon was so loose.
They produced the baseline, but it was a baseline which left a lot of room to discuss and argue about christian doctrine.
I mean as you say unity through exclusion, they could only exclude so many at a time :P
Firstly the OP doesn't make a distinction between the christian faith and the christian church. As far as I know there is only one christian religion, with many churches or traditions.
The as for the great schism (or eastern schism) maybe. The Catholics and Orthodox are fairly similar in ideology and tradition and the "roman" structure of these churches could make a reunion work. While a short term reunion is highly unlikely, long term.. well still unlikely but not completely impossible. This is also the reunion the popes are always calling for (together with the church of England) and under John Paul II the catholic-orthodox relations became a lot better.
For these same reasons there is no way that the western schism is ever going to be mended as there is very little structure and a very different tradition in the protestant churches. And while relations between these churches arent bad the vatican doesn't really seek rapprochement with their bible reading counterparts. ^^