As mentioned previously, I've been kind of grinding away on a fantasy variant of Mongoose Traveller for a while now, as an alternative to d20-based systems. While 5e looks promising, I'm not quite willing to wait that long. I've only recently started making any real headway, so here're a few bits and pieces that I think I've nailed down.
There's been some debate over Traveller's array of ability scores in our current group, with people favoring d20's. Without attacking the merit of some of d20's distinctions (as Roger does at Roles, Rules, and Rolls), I'd like to provide a defense of Traveller's stats. While Traveller's ability score set, of Strength, Dexterity, Endurance, Intelligence, Education, and Social Status, isn't perfect, it should work well with a few tweaks. First and foremost, Intelligence needs to be renamed. As it stands, it causes all kinds of confusion with D&D players who think it does what Edu actually does. Two promising candidates are Instinct and Intuition. Instinct is defensible for the sensory and deductive aspects which fall under Int in Traveller; the hairs on the back of your neck prickle up, or you just "have a hunch." Intuition works nicely also; for tasks where you're not that familiar, you run on being good as guessing and just an innate understanding of how things work, rather than a formal knowledge. It does have the problem, however, that it abbreviates to Int, rather than Ins as with Instinct, which may, again, cause confusion with D&D Intelligence.
Education is also tricky in a fantasy setting, where generally formal education is confined to the nobility, and therefore should likely be strongly correlated with Social Standing. However, I think this can probably be handwaved away with a reasonable bit of imagination; a character with high Edu but low Soc may be a former noble who was disgraced and removed from his station, or reasonably even a medieval-style monk or friar, with very little social clout but quite a bit of book learning. So that's not a huge problem.
Finally, Social Standing has caught a lot of crap in our game for being useless and nonsensical. This is, to some degree, true in a more-or-less egalitarian science fiction setting. However, Social Standing is a beautiful tool for modeling the social system of medieval society. The king isn't just going to listen to some uppity peasant, even if he happens to be a competent swordsman or very persuasive; it would be unseemly, and betoken weakness. Mechanically, this can produce barriers to entry, as with "Look, they're not going to let you join their knightly order because your father was a dirt farmer, and you haven't done anything noteworthy enough to earn yourself a title. You need Soc 9 or more to get in." However, on the flip side, Social Standing is also a great mechanical consequence mechanism for player actions instead of gold or XP. Caught red-handed and branded an outlaw? Set Soc to 2. Slay the dragon, save the kingdom, and enjoy the accolades of the people? +1 or +2 Soc. Gain a noble title and lands, or become the archbishop or archmage? Set Soc to 10ish. Marry the princess? Set Soc to 12. That kind of thing. Thus, the problem isn't with the Social Status ability score; it's that we've been using it wrong. Hell, you could even base starting gold off of starting Soc; that'd make sense, and make it useful.
I also dabbled with the idea of a Piety score to run divine magic, but I think I've abandoned that because it would be a real pain to keep track of, and would vary with deity. Speaking of magic, that's next in this series, I think...