I'm really surprised that I haven't found something like this out there, but after re-playing Symphony of the Night with Gemini's translation, I would really love to see a difficulty hack for it.
I might not be speaking for everyone when I say this, but I found the game to be pretty easy. I think making some enemies hit harder would be helpful, especially in parts of the first half of the game. Enemies in the 2nd half seemed to die way too quickly, especially after you got access to certain pieces of equipment (while we're on that subject, the Crissengrim[sp?] ought to be banned). Overhauling the resistance system as well would be great--elemental resistances are super effective, to the point that the elemental absorption equips aren't really necessary. The Holy-resist armor literally cancelled out most every attack from the mid-game boss, who should have been much more of a challenge (for logical and canon reasons).
IMO, I think trimming the item list a bit (and making the equipment list more noteworthy/useful) would make things so much more interesting---The Holy Rod literally tore everything up from the beginning until roughly endgame. I was still using it basically into the final battle, and ~80% of the weapons I picked up/bought were well outclassed for the second I picked them up. Maybe cutting down the effectiveness (or cutting it out completely) of the elemental resistance/damage functions could make certain items more valuable too. And seriously, the food items generally need to just go--most of them heal less than potions, and they take up an inordinate amount of inventory space all the time.
Also, unless I missed something, some kind of auto-sort function would be amazing for the items.
As far as the bosses go, I think the 1st half's bosses were actually pretty decent, but most of the 2nd half's were really weak, possibly keeping in theme with the laziness of the 2nd half's setting concept. If we can't have a redo of the entire 2nd dungeon, I would definitely take a redo of the traditional monster movie bosses, which were all super easy anyways. Beelzebub and the "King of Hell" were really the only bosses that posed much of a problem (and required somewhat skillful strategies to win), but even on them I never died. Doing something about the final boss would be lovely too, because I took him down pretty quickly with relative ease.
And if this type of hack exists already, please, point me to it!