There are a fair number of concepts I would love to use for a campaign, but I also would not mind turning some of my past one-shots into campaigns:
Paranormal 1840s Vienna: A supernatural investigation society set in Vienna during the 1840s, or at least before the 1848 Revolutions. It's crazy how many peoples and present day countries/territories were part of the Austrian Empire during that time: Austria, Hungary, Transylvania, Czechia, Slovakia, Slovenia, Croatia, Poland, Lombardy, and Venice. So you are basically throwing the supernatural and paranormal in the mix of a multi-ethnic empire that was ready to tear apart with nationalist movements. So throw paranormal monsters and investigators from all around Central Europe into the capital city of Vienna - using real city maps of Vienna from 1840s (and players familiar with present day Vienna) - and have at it. My major issue has been with deciding a system: Dresden Files Accelerated (as per the original one-shot), Vaesen (YZO Engine), Monster of the Week, or Urban Modern Fantasy (essentially a modern, urban fantasy version of Dungeon World).
Fantasy Renaissance Not-Venice: I ran a one-shot of Fate for an ex's friend that I set in a fantastical version of an off-brand Venice in an off-brand Europe with magical elements. The original one-shot involved a young, black sheep scion of a noble family (player 1), his duelist surgeon retainer (player 2), and his associate banker/treasurer (player 3) trying to gather the ingredients in the city for a spell for his older sister (a priest/exorcist) so she can seal a portal to the Spirit World in a monastery on an abandoned island that his family had acquired for their shipping business. Demonic spirits of rage and terror were bleeding into the world and feeding on the spiritual energy of the deceased monks who had died in a siege from a rival city-state about half a century earlier and the spirits were possessing island monkeys and raising undead. It was a load of fun, but I would love to go back to this setting for a proper campaign.
It was inspired by The Lies of Locke Lamora, Borgias/Borgia, Medici: Masters of Florence, and Assassin's Creed 2, but also with magical elements, such as the city never flooding due to a pact made with the merfolk who live in the lagoon, a Spirit World, and alchemy. As you can't understand Medieval or Renaissance without a Church, I needed a RCC analogue, but I also wanted to avoid tip-toing around Real World Religion, so I created the Church of Angelic Virtue. It was a Church centered around venerating the ethics of seven angels/spirits who are believed to embody the seven virtues that most reflect the Divine Will of the Creator. If I would re-run this, I would naturally consider Fate, but I also lean towards running it using Cortex Prime, especially as I could make the 7 Virtues into a trait set.