So, for those not up to speed on Games From Folktales, as part of #City23 I've written a 60 000 word supplement on Venice which is up on my Patreon for paid members and will go out as a "pay what you want" thing once the ArM5DE license is live. Onto the next thing. In the past I've tried to run votes on the GFF Patreon, but the numbers have been too small to see what the Ars fanbase wants, so here we are and here's what I can start in the near future. Pick one and realise it may take as long to deliver as Venice did. 8)
Your options are:
I rewrite Venice as a Domain of Dread for D&D 5E. This is likely the most lucrative for me, btw. I know its not helpful to the larger Ars Magica project, but I now have some gorgeous maps of the Ducal Palace and I can fill them with monsters. Alternatively, a Domain of Delight also works.
Post-a-day in November creating vendors for the faerie market in Venice's Merceria. Essentially it is an added chapter that clicks into the Venice stuff already in the book. The core text here would probably be the faerie market we already have in Thrice Told Tales.
A book like the Venice book, but based on the island of Sicily. The core texts will be "The Normans in the South" and "Beautiful Angiola".
A book that works as an entry level work for people wanting to design their first covenant in Cheshire, which is in North Wales. Core text is the Christina Hole "Folklore of Cheshire". Wales is not well described in Ars Magica and I have all the stuff I need. Also David is off writing intro level characters and stories on his Patreon so it could act as a sort of epiphyte project on that. (which is a reminder, after the CC-ish license comes due, there's going to be a lot of potential for people to say "Well, I liked your X, but it needs more Y" and do a product. Some of the games on Twitch are single-page.
I know it is tempting to say "I'd really like you to write Z instead", which is fair, but I know I can do these five.
I vote for Sicily. You’ve already done the Cornwall book for Stonehenge and you’ve been talking about writing up Sicily forever.
You should also, on the side, do the Domain of Dread. No one in this forum is going to vote for that, we’re not the target audience. But you deserve some return for your considerable in-Venice-ment.
Completely useless answer: The one that you want to do, the most.
Makes sure it's something you want to do, not something you'll spend more time hating having to write, than actually writing it.
Personally, I'd probably prefer the Sicily book, though the Cheshire one could be interesting as well. I'm assuming there will be at least one cat with invisibility powers, mind you
Sicily is an extremely complex area to do justice to in 1220, when it's the favourite abode of Frederick II, King of Sicily and Holy Roman Emperor, with his retinue of christian and jewish and muslim sorcerers and artists and concubines and scholars. In this sense, it's probably one of the least self-contained areas of its size in Mythic Europe. So, I think it's a bit too ambitious a task for the scope of project you are proposing; it's about the scope of a minor Tribunal such as Hibernia or Loch Logean. But that's probably just me. Then again, I see that the core texts you cite are:
a) One about the Norman conquest of Sicily (1016-1130) so about 1-2 centuries before 1220.
b) One about Sicilian folk tales, many of them from well after 1220.
So maybe I'm getting what you are doing all wrong?
If I had to make a choice, it would probably be what Bob said:
I'd love to see the Post a day for the Merceria. I think that would be a really nice addition to the book, and doing it as a 30 days project might also act as a teaser for the book for those not subscribed to the Patreon.
What I would enjoy/use the most: Mythic Cheshire*. Lots of people run games set in the Stonehenge tribunal either because they are locals, or because of the amount of English-language resources available.
What I think you should do: Venice as a Domain of Dread. Exposing more roleplayers to the idea of a historical-fantasy setting, and probably making you some dosh? Seems like a strong choice.
*As another native Brit, I object to it being classed as "in Wales", but this has already been noted.