Well in my opinion I think doing that would make those stances at least more usable and accessible. Games with many unique characters or any game in general tend to have the strongest build or the strongest melee/magic/shoot character. The longer the game the longer the players can discover which is which and that goes the same for which raids profit more, etc.
It really isn't new that GE players only use certain characters, hence their weapon type, depending on the time they were meta. For example,
Melee: Triple Claude/Adelina on Twin Blades / Triple Fengling > Triple Grandma > Triple Selva> release of expert stance ruined diversity > Red Hair > JD > Lynn > CNL/Leona/Judith > Doom/Sakura/Ru
The only place I can honestly say that stances played a huge role and where characters are a lot more diverse was during wxpvp fix era before new characters that can 1 hit on fix mode were released (Pre-nerf CSC/CC/Leona/CNL/Daisy/etc.)
Knowing which stances provide high block and evasion played important parts and where unique characters such as Illusion (she is an evasion melee that can ignore evasion as well as block and can debuff with "love"), Jane/Barrel (shooting chars usually get wrecked by melee block chars but they have skills that can ignore block), RE/Hellena/Wiz/Darkness (when debuffing is a thing and high aoe burst with invi for ambush) etc.
They kind of differentiated weapons through their stance like most rapier stances centered on high evasion or anti magic and daggers being used by invi chars but I do get your point that most tend to blend with each other like swords with sabres.
The only perk in using a non-meta char/non-meta weapon type is that you get famous if you do it well on pvp, pve not so much hope there.