If you please, I would like to request some assistance in idea-building. I have a minor plot point in a story I've been working on, and in order to carry it out as it presented itself, I need to come up with a subject about which open and heated public debate, and possibly covert warfare, among the nobles of Canterlot could occur.

1) The story setting for the background event is ~300-400 years before the present.
2) The issue in question ought to be a political dispute, of at least some seriousness.
3) I have one particular noblestallion who will be the public face of his side of the dispute. The issue needs to be pressing enough that his enemies feel justified in poisoning him (as in, to death) in order to prevent his voice from being heard... which does run somewhat afoul of the Equestria we currently know.

Given that this dispute, whatever it may be, will color Equestrian history for some time going forward, I do not feel completely confident in just making something up out of thin air. Does anyone have any suggestions for what the topic of the dispute might be?

A historic example could be the imposition of high income and land taxes in England during and after World War 1. These forced much of the landed aristocracy, to break up and sell off their estates. People whose fortunes were based on generating income from manufacturing or finance could pay the taxes. Those whose income were generated by what they could get from tenant farmers could not. See "Revealed: why the real Downton Abbeys went into decline after WW1"

I could see a similar issue arising with the Equestrian aristocracy. If there was a new land tax that was about to be proposed, and a faction of the aristocracy saw that it would lead to the eventual elimination of their great estates and thus their power and place in society, then they might take drastic action to keep the tax proposal from coming to the fore.