There are so many mythic locations that sound perfect for adventure and campaigning and story that its hard to choose.
Though socially and physically sites in England are perfect, the political aspects of England and its king make it hard to choose that location if some sort of ties want to be had. I kind of wish there was an updated tribunal book for England, though in all honestly the one that we have from the previous edition is still mostly good to use with only little change or additions needed. (Unlike the Italy one for example, which is demons and infernal and evil everywhere [don't look under your book, a demon lurks there] and is pretty much useless for the current Ars Magica edition.)
Hibernia is an interesting location, it has a lot of story potentials and while I was going over the book looking for sites a number of possibilities came to me. Which I consider to be a sign of it being well written.
Going over it again and again I cannot help but see the site of Candia (Crete) in the Theban Tribunal. The island nation itself has a magical and faerie aura that could easily be used to provide magic to a covenant site. There is already a pretty interesting being that could be utilized as a patron, Talos. There aren't covenants on the island, which means that players don't feel like they are stepping on the toes of canon information and need to make massive changes that might have repercussions that they first didn't notice. It also has a pretty awesome vast ravine (the Gorge of Apollon) which would be a perfect site for a covenant, just simply perfect. (I can already picture the structures of the covenant carved out on the cliff faces of a segment of the ravine with bridges connecting both sides.)
What is also quite interesting is the political situation of Candia. Right now its controlled by Venice, but the native Greek peoples of the island hate this fact (both in-game and historically) and so they are basically constantly harboring individuals who are planning and seeking to free themselves from the 'iron fisted yoke of those imperial warmongers, the Venetians' (my words, lol). If events play out as history shows, in nine or so years a rebellion breaks out. Of course in game such actions are influenced by the players which means the event might have a different ending than they otherwise would. Furthermore, historically the first rebellion was propagated by two brothers, who lost, but the fact is I couldn't find much information on them so basically I can do anything I want with them. Including making them awesome cool guys that are personally exactly what I want them to be in a way that is fun and awesome; which has a lot of potential.
I need to figure out the whole family situation of my character, but that is my own mission. Though I kind of want him to be part of some really big family, with uncles and cousins in various fields of endeavor throughout the nearby lands.
One final question, though this goes a little off the point of this thread - Theban Covenants need a purpose in their founding, which is kind of hard to figure out what purpose that the one founded by the player on Crete would be about.
PS. My characters driven goals are twofold, one to establish a school of magic where those with the Gift can learn all they need to learn (aka Hogwarts style; I plan for certain breakthroughs to occur to enable group learning of magic and the removal of penalties for the Gift in teaching other Gifted) and the formation of a kingdom for his friend, the lord (this is mostly because the idea of using magic to create out of basically nothing a vast territory is really really cool; though I might have to put this idea aside as its basically its own thing, hehe).