Of course, it's very easy to snipe from the sidelines, so since I've prepared the noose for others, I might as well start some work on what I'd like to see in such a setting.
1st Principle: Historical inspiration over historical accuracy
Don't be a slave to history; tweak, alter, and ignore history to make the setting interesting -- which is good considering that the history wasn't that pretty (especially for us Filipinos).
There are some cultural and behavioral patterns that seem to repeat themselves a lot in our history, and -- big and small -- they should make their way into the setting. Otherwise, why'd we even call it one? Some things that come to mind are:
- religious tensions and evolutions - before the Spaniards came over to disseminate Roman Catholicism, religion was a mix of animist beliefs, the muslim contingent (which was never completely eradicated in places in the Southern area), and whatever else came over with the traders from nearby countries (China, we're looking at you); this kind of religious diversity, openness, and intolerance must be an element.
- cultural tensions - as an archipelago of 7,100 islands (give or take given the tide) with mountain ranges and valleys, and a host of caverns and other natural barriers and natural resources, there were a lot of varied clans/tribes/alliances that developed their own languages and customs, warred against each other, and even "betrayed" one another when the Spaniards came to invade; this kind of factionalism and divisiveness must also be an element.
- adventuring tropes - with D&D as an inspiration, we look for adventuring sites. Wilderness adventures are an obvious choice, but cave-based "dungeons" may be done as well. City-based adventures would work, along with the smaller towns ravaged by a single creature or a small horde of 'monsters' attacking the town. There's no shortage of local creatures to fight: higantes, aswangs, pugos, kapres, tikbalangs, manananggals, engkantos, etc. Sadly, most of the big structures were built by the Spaniards (that we know of), so we'll have to be creative if we want some kind of break from this disparity. Magic systems will be interesting -- various types of local witchcraft, and local bits of color like anting-antings and oracions; these must be represented in some way as well.
Furthermore, there are other RPG-related concerns: game balance (or lack thereof), abilities that can consistently be represented by rulesets, thematic decisions, the ability to inspire a sense of wonder, and so on.