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I personally like the Palladium system, Rifts and Heroes Unlimited in particular. I would give those a try. GURPS is also worth checking out.

"Custom" spells would be quite good. I don't see any reason, other than cost, why not to include such a feature. Also I think that something like metamagic(from D&D 3/3.5) would also be an interesting mechanic.

It should vary from culture to culture. I could see a female dominated society that regards males as little more than cheap labor and walking "sperm banks". While there could also be a society that sees genders as relatively equal.

Limited ammunition please. However, arrows(and stones if there are slings) should be relatively easy to craft with from variety of materials that can be found in the wild.

Lets see... Serial Killer Kingpin Bigot Pirate Kill family members Hermit Vigilante Slaver Commoner Pretty much any other "unpleasant" thing that other RPGs restrict so they don't offend anyone.