What I think about scythes in this game:

The only time I've seen one, at least in FF XIV, being effectively used as a weapon, had it in the hands of a Necromancer.

What am I suggesting?

Having played FF XIV for about two months now, I can't say anything as an "expert FFXIV Player", but I've seen other games of the franchise and I've seen Necromancers here an there.

Necromancer was even meant to be a class to FF XI - they just discarded it to give birth to Puppetmasters.

Necromancers, at least in FF XIV, have a lore that goes stray from the common Black Magick line of though. While Black Mages were mostly users of void magick - which would be something like occultism in our world - Necromancers would be, in fact, twisted White Mages in being.

How so?

As you go through the lore, you face a handful of effectively important Necromancers (aside from your everyday cultist lunatic). One of them even use a scythe as a weapon, even though it's barely used to melee swipe someone, rather being used to focus a grim white cast.

The other one practically twists the white elements in order to manipulate the flux of life into things that shouldn't be really alive anymore.

What then?

Differently from your everyday Black Magick, Necromancers in FF XIV are quite a warped thing, and could fit easily as a support class (damage conversion mechanic). They could use scythes (as some sort of warped staff) as their main weapon, even without the need of ever using it to swipe anything on range, it's thematic, and completely clean design imo.

Other than that, my narrow creativity can't really imagine any other classes wielding scythes as effective weapons.

If you want darkness, have a DRK.
If you want black magick, have a BLM.

If you want plain madness, add NCR to the game.