I know three ways to boost crit range beyond the "double crit range, doesn't stack" things. However, you can't do them with a fighter.
Streetfighter barbarian: +1 crit range
The streetfighter barbarian gains +1 to his crit range when attacking flat-footed opponents or charging. You need 7 levels of barbarian to gain this ability.
Arcane duelist: double crit range
At level 7, the arcane duelist gains False Keenness, and 1/day/level can make his weapon keen. This explicitly stacks with Improved Critical and keen weapon property, but not spells. It's 3.0 material, but if your DM allows Disciple of Dispater, he should allow this too.
Unfortunately, it's difficult to enter - you need +6 base attack bonus and the ability to cast 1st level spells. You will want to enter with something like duskblade or hexblade, who gets arcane spellcasting and full base attack bonus, and even then, you won't have this ability before level 13.
Psychic weapon master: +2 crit range
At level 7, the 3.5-updated psychic weapon master gains Improved Critical, but if he already had Improved Critical, he gains +2 to his crit range. This increase only applies to the crystal weapon that the psychic weapon master has bonded with, so you'll need to have a crystal scythe.
Psychic weapon master is also difficult to enter - you need 3rd level psionic powers, base attack bonus +5, and a bunch of feats. The fastest way to enter is as a 7th level psychic warrior, meaning you need to be level 14 to gain this ability.