Duchy of Lionsborough

Just under two centuries ago, the Archmage Alexis Worldstrider was travelling across the planes when they came upon a unusual convergence of mana flows. After finishing their adventuring career, they settled in this mana-rich environment, forming this own private demiplane. As their ambitions grew, various employees, contractors and hangers-on settled in the demiplane surrounding the tower, eventually growing into a good-sized town.
Alexis didn't really care about government (research was much more interesting), so they were more than willing to delegate responsibility to a "town council", which morphed over the first couple of decades into a proper representative government. Alexis, as "Duke", became a constitutional monarch, performing ceremonial duties and keeping an eye on corruption levels in the government but otherwise being steadfastly apolitical.
Alexis stepped down forty seven years after the formal founding of the town. One day they invoked a little-used power to summon the parliament, announced they were stepping down, and introduced a successor as Duke. This custom has continued, with the existing Duke nominating a successor. The Dukes are notoriously close-lipped about the process by which they choose a successor, but all have been generally benevolent and hands-off.
Now, Lionsborough (Alexis' emblem is/was a lion) is a thriving interdimensional hub. It is almost completely urbanised, with a good amount of parkland and gardens and other green spaces. Architecture tends to be white marble, with graceful butresses and gothic arches. The weather - and entire ecosystem - are maintained mostly by magic. Food and water (and removal of waste) is performed by magical conjurations (Create Food et al). About one tenth of the native population have Sorcerous Empowerment at some level, far higher than normal, but not enough to seriously stratify society. The ubiquitous access to magic, or at least magical items, means that basic needs are easily met, forming a post-scarcity utopia. 
The town is at an equivalent of TL 8^, with magic replacing technology where appropriate. It is advanced to TL12^ in interdimensional transportation, having many portals to other planes and dimensions, and being easy to reach with Plane Shift spells, or equivalent psionics or superscience.
== Adventure Seeds==
*What happened to Alexis? Their private chambers in the Tower are still sealed, and the rest of the Tower is now a museum. The PCs are employed to find out.
*How are successors actually chosen? Is some sort of test or trial involved? A journalist believes the current Duke has a secret, and pays the PCs to investigate just how they became Duke.
*The risk of invasion is low, but any army capable of travelling between dimensions is a force to be reckoned with. Can the PCs come up with a plan of attack/defence?
*Is the town really as utopian as it seems? If something appears to be too good to be true, maybe it is!
==References==
*Realm Management examples post #86