Grognardia has another thought provoking post here about the Dangers of Imitation.
It is good post but it raise the questions about what paths we take moving forward. Will the general history of the RPG Marketplace unfold in miniature? Or will we explore avenues that were ignored the first time around?
There are a few avenues we could exploit that allow allow for new types of products but yet still remain firmly in the Old School Camp.
The first is the Points of Lights/Wilderlands format. Not so much the whole isolated outposts of civilization thing but the idea that setting can be presented compactly and usefully by using a numbered Hex Grid. I would like to see these become as common as adventure modules.
The fact that I had a list of numbered locales to detail really helps to keep my focus. With a regular map you can ramble all over the place with really no limits what details you include aside from the number of pages the product has. This is similar to what people describe when fleshing out a dungeon level with numberd rooms.
The second is what I call the Tekumal model. AD&D had this with Oriental Adventures and the Dragonlance Hardback, and a more recent example is Supplement V Carcosa. The idea is that the author has developed a world with a specific and adapted D&D to model that world. Classes, and spells change but the system is still recognizably D&D.
Because the core of your rules remains OD&D other referee may find useful bits. Just like in the earliest days when referees played in different campaigns and took the bits they liked back to their own game. If you want to play in that setting then the rules work together to give a different experience than the core rules. By adding to the variety of OD&D then we keep the market fresh and interesting.
The last avenue is what I call the gonzo factor. Lead by Mutant Future and Encounter Critical these books are a mishmash of elements to create unique and fun games. While this went on in the early days (Expedition to the Barrier Peaks) I think this will be most enduring legacy of the Open Game License. The OGL spirit naturally lends itself to kitbashing rules and ideas together to make interesting games.
It doesn't have to be silly either for example this path can lead to supplements allowing OD&D to be used for Planetary Fantasies like John Carter of Mars.
There will probably be more ideas coming down the pike as well as more adventures, monsters, and magic items. But these are three ideas I think that authors can use to create Old School Products that are not just imitations.
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