Interestingly, Hungarian history remains largely unexploited in gaming even within Hungary. In the 1980s, when AD&D arrived in the country, and in the 1990s, when gaming spread beyond a narrow hobbyist audience, the American game worlds were seen as more interesting (and probably more exotic! ) than our own. The most popular local game, M.a.g.u.s. (which is derived from AD&D, but differs very much in its core assumptions), is set in lands most closely resembling mediaeval Italy. It has remained a curious blind spot all through the years, aside from one badly regarded Shadowrun supplement, and Hussar's Strike, a mini-game set during the Turkish wars. A friend of mine is running an OD&D campaign set in a land influenced by 19th century romantic epics (essentially proto-fantasy!), but that's one home game.

Mediaeval Hungary does, however, offer some interesting features for an RPG treatment:

• While not fully "points of light" in the American West sense, Hungary was much less densely populated than France or Italy, with only about 2.5 million inhabitants in the mid-14th century (France had 20 million). This makes for much smaller settlements, and a lot of room for adventurers and monsters. On the other hand, the great mediaeval famines had never struck Hungary: population never grew enough to hit a growth barrier.
• Much of the centre of the country was wetlands and dense forests, where villages and small monasteries would often be well hidden. On the other hand, the mountain regions on the edges were very underpopulated as well, save for specific border guard populations (the ancestors of the modern Székelys), and miners. Since Hungary's wealth was in gold and silver, mining towns flourished, mostly run by Saxon colonists ("Saxon" refers to a set of privileges, not necessarily precise origin). Some of these towns to the north were pawned off to the King of Poland, who would have small "bubbles" of territory within Hungary.
• Royal power fluctuated significantly over time: there were periods with very strong central power, and others characterised by the almost complete dissolution of this authority, and de facto rule by major landowners, who were almost kings in their own right. Until the consolidation brought by the Anjou family in the 14th century, this would go from one extreme to the other within a single generation. Both orderly fantasy feudalism and outright civil war are potential backgrounds.
• There are, ironically, no notable mediaeval vampire legends associated with Transylvania: the region is mostly associated with ghost stories and the faerie (it is sometimes called "The Fairy Garden", not necessarily in a positive sense, but as a wild and eerie place). The close analogy for someone brought up in the USA would be Scotland.
• The Mongol invasion left many ruins behind, a good place for dungeoneering. Roman remains were also present, although universally ruined, since in the period between Rome and the Kingdom of Hungary, the Carpathian Basin had been mostly reduced to wilderness, with the occasional petty kingdom or short-lived nomad empire - but certainly not kingdoms able to support the former Roman cities and fortresses.

In the section on "The Continent" and the timeline of future events, the Dark Albion book details quite a bit of information on Hungary during the time of Mattias Corvinus.