1) A wide variety of weapon types works well when the game is designed around the possibility of using any of them.

Divine Divinity had Daggers, Swords, Axes, Maces, Spears, Bows, and Crossbows all as separate types. Guess what? Only swords were worth investing in - not just because it took until the Good Old Games v1.4 release before all the non-sword mastery skills were changed to percentage bonuses instead of fixed bonuses. The problem was that there were three great swords for every one of any other type. I think there were maybe two magical spears all game, and hardly any magical maces, one or two magic bows, no crossbows. If you want to make weapon types, remember to have a fairly even distribution of valuable ones, and not 3 awesome swords per awesome other weapon.

Adding in nun-chucks and a hundred other weapon types might sound fabulous, but how long would it be until the player finds any of them? How many unique upgrades would they be able to find for nun-chucks? If nun-chucks and their two-dozen cousins are added to random loot tables, that could frustrate users of more conventional weapons, because there are only so many places where something could drop, and instead they keep finding lots of weapons worthless to them.

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2) Doesn't OS have like 4 out of 6 skill categories dedicated to magic?

That was the last I'd heard - that there were only two non-magical skill sets. It sounds like this game is based more around elemental damage/resistance than slashing/thrusting/piercing resistance (which is fine, that didn't work so well in Beyond Divinity). I think a simple "Imbue Weapon - [Element]" spell, which imbues the current weapon with 3-10 charges of whatever element the spell was cast in (probably one of the skills should be on each elemental tree) would be a good idea.

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Okay, with that out of the way, more on topic, weapon types.

3) With two players, distraction, flanking and getting behind an enemy becomes much more viable, so DAGGERS designed to work with a Backstab skill are a good idea - one player distracts and tanks, the other flanks and stabs. (Maybe an invisibility spell lasting 3-5 turns would work as well.)

4) SPEARS because some Skyrim and Oblivion players just can't let go. Spears work especially well in turn-based games because with careful movement and positioning, they should have enough reach to hit a melee enemy who ran out of movement a square too far to hit the player. You know what would be helpful? Being able to see the range of an enemy's movement so you can know how far to stay away and kite them. And of course, with spears, you can Skewer two enemies together, and maybe that would let a spell affect both.

5) MACES because this game has a slip-and-fall mechanic, Maces could have a small, but helpful chance to knock down whoever is hit, making them lose one turn. Note that this could happen to the player as well, so it could be dangerous.

6) SHIELDS - specifically, shield bashing and shield charging. Perhaps at the end of your the turn you could also switch to a defensive stance to lessen damage / evade attacks, and Shields increase the amount of damage reduction.