The Nine Ringed Broadsword is one of my favorite PF1 weapons and there are some specific examples of it in my converted AP. So I wanted to convert it, probably using either the scimiar or battle axe as the base example and just adding the monk trait.
I lean to the former option because of the weapon's flavor text, but it occurs to me that Forceful is REALLY strong for monk who can throw out lots of attacks. On the other hand, it's not any crazier than a ranger dual wielding scimitars I suppose.
What do y'all think?
I think Forceful would move the die type down, like a Scimitar. That seems to be a normal pattern, that these weapons make up the damage and they break even, but have some other perk that maybe shouldn't have a higher die type (like one that adds to attack, i.e. Backswing where the Great Club does less than a Maul)
Trading Sweep for Monk seems balanced.
As you mentioned, a Flurry Ranger could very well be using scimitars...especially if escaping the Underdark. :)
There do seem to be specific balances to the worth of weapon traits and maybe even type of damage, but I haven't seen that tackled. I'll guess there's a formula or point-buy method.
So 1d6 Slashing; Forceful, Monk
I don't think forceful is that big of a deal as long as you don't also make the weapon agile. Falling stone is notably a forceful monk attack style that is an available option already. Just make sure that your new weapon is demonstrably weaker than that weapon. Castilliano's suggestion is probably just fine.
The Monk property doesn't seem to actually factor in to the balance of a weapon and is just a way to restrict Monks to the most thematic weapons - the Bo Staff compares favorably against other martial 2-handers even if you're a Fighter, for example.
Guisarme: d10 S
Bo Staff: d8 B, Monk, Parry
Both have Reach & Trip
Given how rare Parry is and it giving an actual numerical bonus, that's probably the balancing factor for the lower damage die. The other Parry weapons are 1d4! (though have lots of other traits too)
Monk & Bludgeoning make the weapon better, but hardly compare.
Longsword vs. Temple Sword is another example implying the Monk trait is mostly chosen for flavor, yet I doubt you'll see a d10 or d12 weapon w/ the Monk trait unless it required a Stance or similar cost. So the Monk trait becomes a useful gate to keep Flurry reasonable.