Also, I'm wanting to know if the game is specific to one particular nation or region of the European Middle Ages, or if it can be useful for any Medieval European campaign? I know Dark Albion is specific to England at the time of the War of the Roses, but I don't know the full details behind Lion & Dragon.
Unlike Dark Albion, L&D doesn't have a lot of explicit setting material (that's what Dark Albion is for). But it is set up with the idea of playing in England during the War of the Roses, so you could say the ideal place and date of a L&D game would be England in 1455. Indeed, some of the flavor text on things like the alchemical substances presume that it's after 1453, based on people it names and the presumption that Constantinople has already fallen to the Turks.
But the material in L&D is open enough that you could play anywhere in the late medieval/early-renaissance period, anywhere from around 1200 to somewhere a little after 1500. You could run it earlier, but would have to make some slight changes.
Next, I'm wondering what supplements (if any) would any of you recommend for L&D down the line?
If you want to play in the full-on War of the Roses era, then obviously Dark Albion. If not, you should seriously consider Cults of Chaos. It is basically a generator of evil groups (cults, covens, heresies, etc) with everything you need for playing tons of adventures.
Then, check out all the RPGPundit Presents series that are Medieval-Authentic. There's new magic expansions, adventures, and other interesting setting or gaming material. Most of these are also smaller books, which are quite inexpensive.
Finally, I'd like to ask Pundit himself what he thinks of a "sequel" to Lion & Dragon/Dark Albion that covers the 16th and early 17th Century? I would love to buy an OSR game that does for the Tudor and Jacobean eras what Dark Albion and Lion & Dragon do for the Wars of the Roses and the Late Medieval period.
I could see maybe doing something about the Tudor era. But I'd be more likely to go backward, rather than forward, because the Tudor period is really not medieval at all anymore, it's full-blown Renaissance. The medieval period of England pretty much dies with Richard Crookback at Bosworth.
So if I were to write another big sourcebook for L&D it would probably be one called "Deus Volt" and would be set in the crusades.