If the setting is going to be "vaguely western european medieval" then, please, please, please
1) have some in-game "history" that believably justifies feudalism other than the ever popular "its cool" or "we can't think of anything better" ones ...
2) Have an economics system based on some believable distribution of precious metals ... and not with ~37-45 gram gold coins (i.e. 1/10th of a pound, presumably avoirdupois, but possibly standard) ...
3) Work out that there's a correlation between the types of weapons used in a period and the most common armour available, and that this does NOT mean that weird **** weapons from all over the history of creation are likely to exist in the game "here and now" as a result ...
4) Realise that societies in the real world, even those supposedly static ones, do NOT remain technologically or socially static for many thousands of years if they resemble western european ones in any substantive way ... so no multi-thousand year long "Third Age of Social Stasis", for example ... you'd be hard pressed to get a "TAoSS" of a century or so, and only if you didn't look very hard at the history of the era ...
... and more, so that it makes some sort of sense.
Either that or take a "toolbox" approach and discuss the various possible building blocks of a "medieval like" society and let individuals make up their own ridiculously unrealistic worlds :lol:
Phil