Can anyone with a better knowledge of medieval practice comment on the opportuinity of naming Duchies after families?
If I look at this list:
http://en.wikipedia.org/wiki/Peerage_of_England
I see that Barons were not named after a piece of land, but simply after their family name (Baron Cromwell, Baron le Despenser, etc). However the titles of Earls and above were always tied to lands (although often named after a general area, and not a town).
In BM Duchies are the only free-form titles, so we can't copy the practice for Barons, but technically we could for Duchies.
Would it be acceptable to name a Duchy after its first owner (e.g. Duke Bedwyr of the Duchy of Bedwyr)? That could bring an element of inheritance in the game, without needing any mechanics change.
I think you are confusing the mode of address with the name of the title. This is a very easy misunderstanding to have because they were often switched around.
Anybody who held a "lordship" (basically anything from a Sire-dom on up) could be referred to by their geographic title. This included dukes - you can see in Shakespeare (or in Holinshead, which was Shakespeare's main source for most of his English histories) where they'll call the Duke of Norfolk just 'Norfolk.' Technically Kings could do this, too, since they were considered the human personification of their realm.
Anything smaller than an Earl-dom (roughly, a county) was typically not going to be a big or important enough piece of land that you'd necessarily want to change your mode of address to suit it, particularly because it would mean giving up your family name. If you go back to Hastings, though, when Willie the conq handed out a lot of lower-level titles to his knights, the baronies or lordships effectivley got named after them, rather than vice-versa -- this is actually the case in my own family, where we have a pile of rubble in the Southeast corner of England that goes back to Hastings but most of us died from the plague.
The other place you'd see baronies used a lot was a higher lord's estate and the surrounds, such that the Duke of Norfolk (place) was also the Baron de Mowbray (the name of most of the early Dukes of Norfolk) and you could also sub-divide the bigger plots so you could have multiple (social) ranks of Baron de Mowbray even if there was only one actual "Barony of Mowbray" - these titles were not really linked to a formal plot of land (e.g. it's as likely as not that there is no place called the Barony of Mowbray) because they were always held by people who had higher titles.
You wouldn't create the Duchy of VonGenf though. The Western Medieval way of handling it would be that Von Genf Dude would get created Duke of Suffolk and from then on only his family, priest, and very, very close acquaintances would even use the name "von Genf." Same reason nobody would ever call Henry VII "Henry Tudor" once he became King or Edward III "Edward Plantagenet" except perhaps as part of a very formal treaty, and even then it would be Edward III du Plantagenet and quite possibly just so he could rub in his Frenchitude to whatever claim he was making on large portions of France (Plantagenet having come from Anjou, or Angevins). In this example, all the younger sons might be referred to as Lord von Genf (socially) or Sir Von Genf (if knighted) or else the Duke might have a bunch of minor lordships or baronies to hand out to his own family if he wanted them to have landed title and not just social rank. In France you had a lot of this with people other than the King having tons of titles to give out, creating many rungs on the feudal ladder, where in England, a slightly simplified description is that the King had all the major vassals (Earls and Dukes) directly under him and only minor lords under them (Seigneurs, Sires, Lords, but not Earls).
