Gunblade...that is all

[Kitru]

All of those are real historical weapons and fighting styles.

You're only right about dual wielding, which I have to facepalm anyone that might think that it's not a legitimate combat style. You can find historical precedent for dual wielding pretty much any combination of sword, axe, dagger, hammer/mace, or pick. The only time I'd consider dual wielding as being unrealistic is when you're talking about firearms/crossbows (e.g. Diablo 3's Demon Hunter).

As to the war scythe, it was nothing like what people think of when you talk about a scythe. A war scythe had more in common with a glaive than an actual scythe used for harvesting. It was only referred to as a "war scythe" because it was a standard scythe converted from a tool into a weapon by beating the blade to be straight rather than perpendicular to the haft (and using a straight haft instead of the curved haft a farming scythe used). Equating a war scythe to the scythes that people expect to wield when talking about scythes as weapons makes absolutely no sense. It's much like the etymological variation of the word "dragoon" as it applies to real warfare (mounted infantry/armored units) as it applies to the fantasy concept ("dragon knight" or some variant thereof).

Gunblades existed, and they weren't just muskets/rifles with bayonets attached. A bayonet was simply an attachment that allowed a rifle to be used in melee combat (as a spear) when firing it was largely ineffective (i.e. in melee). Bayonets were also pretty much always removable; they were an attachment to a weapon rather than an integral part of it, which is what is required of a gunblade. A gunblade is a sword with a gun built into it whereas a bayonet is a blade/spearhead attached to a gun; the primary function is what really matters, and a gunblade's primary function was hacking and slashing (which is born out in all of the FF variations; just look at how Gaius uses his).

Historical gunblades, as in "the unholy marriage of a gun and a sword", did exist but they never saw appreciable use (pretty much all of them were experimental or novelties) because they're laughably ineffective thanks to the huge increase in weight necessitated by slapping a barrel and firing/reloading mechanism to a sword (which are already heavier than most people think). If you want a relevant link, look here instead of at bayonets, since it actually refers to swords with guns attached rather than guns with blades attached.

As such, both scythes and gunblades are terrible ideas for weapons. Yes, they *could* be brought in because there are real world examples (you could probably find an example of someone fighting with a harvesting scythe), but there was also a steam powered (i.e. propelled by steam instead of gunpowered) Gatling gun made in the US Civil War and phrenology was taken as a legitimate field of science for centuries: humans have an incredible history of ludicrous ideas that would make anyone facepalm.

The only way that gunblades are going to be brought in is if you abandon the real world principles and just apply the Rule of Cool, which is what a lot of the aesthetics of the FF series has been built off of anyways (like Yoshitaka Amano's strange belt fetish; who would want to wear clothes seemingly comprised primarily of belts?).

A class built around gunblades presents more of a problem from a lore perspective, given that, as mentioned before, gun-weapons are weapons of the Garlean elite and player characters are Eorzeans, not to mention that classes are themselves based off of guilds. Gun-weapons will work fine as vanity weapons for existing classes (which it already seems the devs are wont to do given the distribution of gun-weapons amongst the named Garlean matching the distribution of weapons amongst the current classes), but I doubt they'll ever be anything other than vanity weapons.