StreamOfTheSky
Adventurer
Well, the Beguiler would be a poor BAB class that would get great use out of such a weapon.... 
It'd then have the same HD and the same BAB as Rogue and Bard, supposedly "secondary combatants." And would be wearing basically the same armors. Except...access to many, many more spells than the bard, rogue's trapfinding, class ability to take shadow spells to break the only "limit" on the class's spell selection, free feinting, and all those other juicy class features....
Wizards and Sorcerers don't need help, either. They can use a plethora of buff/debuff spells to increase their attack bonus or lower the enemy's AC. And bs on the roleplaying reasons to not og into Eldritch Knight. It's an utterly flavorless, generic fighter-mage. It can be molded to fit whatever vision you have of your particular fighter-mage. The reason people don't take EK is the loss of 2 CL (one at level 1, one on a Fighter type class to get the proficiencies). And there are other options to make such a build easier in splats. Abjurant Champion is full BAB and full CL. Spellsword helps to cast in armor. Practiced Spellcaster makes lost caster levels a little less painful.
You want to have a medium BAB without leaving your full spellcasting class at all? Meet Joe the Fighter. Joe wants to go straight Fighter 20, but is wondering why there's no option in such a career path to cast spells like a wizard of half his level. He wants to know how many bonus feats he needs to trade for that.
Back to trying to be helpful instead of just arguing on the basic premise of what you want... If you want a swordsman that uses Illusions, why not just do some Fighter/Illusionist combo with whatever dual advancement prestige class you like the most. Be a gnome, take some racial Illusionist substitution levels. Maybe go into Shadowcraft Mage if you want. It's only 5 levels, you could do that AND Abjurant Champion in a 20 level progression, I think. Look at Spell Compendium. Some of the Illusion spells in there would fit with your vision, and aren't available to Beguilers, except via the extra learning class feature. Bladeweave and Spectral Weapon in particular.
If you want to make a new base class tailored to your idea, you could also use Swordsage as a base. The Shadow Hand discipline has a few maneuvers that fit, but you'd need to reconfigure it or make a whole new discipline more focused on creating illusions, and less on all the other stuff it currently includes (turning ghostly, walking on air and water, ability damage, paralysis....). One of the first level maneuvers create a shadow duplicate of your blade that attacks as you do, so it's harder for the foe to avoid your strike. Mechanically, it lets you roll 2d20 and take the higher value. If you pick the lower value (or roll identical) and it hits, both the real and phantom blade hit, so you deal extra damage. Seems to be along the lines of what you want. And Swordsage is conveniently already a medium BAB class geared towards combat.
It'd then have the same HD and the same BAB as Rogue and Bard, supposedly "secondary combatants." And would be wearing basically the same armors. Except...access to many, many more spells than the bard, rogue's trapfinding, class ability to take shadow spells to break the only "limit" on the class's spell selection, free feinting, and all those other juicy class features....
Wizards and Sorcerers don't need help, either. They can use a plethora of buff/debuff spells to increase their attack bonus or lower the enemy's AC. And bs on the roleplaying reasons to not og into Eldritch Knight. It's an utterly flavorless, generic fighter-mage. It can be molded to fit whatever vision you have of your particular fighter-mage. The reason people don't take EK is the loss of 2 CL (one at level 1, one on a Fighter type class to get the proficiencies). And there are other options to make such a build easier in splats. Abjurant Champion is full BAB and full CL. Spellsword helps to cast in armor. Practiced Spellcaster makes lost caster levels a little less painful.
You want to have a medium BAB without leaving your full spellcasting class at all? Meet Joe the Fighter. Joe wants to go straight Fighter 20, but is wondering why there's no option in such a career path to cast spells like a wizard of half his level. He wants to know how many bonus feats he needs to trade for that.
Back to trying to be helpful instead of just arguing on the basic premise of what you want... If you want a swordsman that uses Illusions, why not just do some Fighter/Illusionist combo with whatever dual advancement prestige class you like the most. Be a gnome, take some racial Illusionist substitution levels. Maybe go into Shadowcraft Mage if you want. It's only 5 levels, you could do that AND Abjurant Champion in a 20 level progression, I think. Look at Spell Compendium. Some of the Illusion spells in there would fit with your vision, and aren't available to Beguilers, except via the extra learning class feature. Bladeweave and Spectral Weapon in particular.
If you want to make a new base class tailored to your idea, you could also use Swordsage as a base. The Shadow Hand discipline has a few maneuvers that fit, but you'd need to reconfigure it or make a whole new discipline more focused on creating illusions, and less on all the other stuff it currently includes (turning ghostly, walking on air and water, ability damage, paralysis....). One of the first level maneuvers create a shadow duplicate of your blade that attacks as you do, so it's harder for the foe to avoid your strike. Mechanically, it lets you roll 2d20 and take the higher value. If you pick the lower value (or roll identical) and it hits, both the real and phantom blade hit, so you deal extra damage. Seems to be along the lines of what you want. And Swordsage is conveniently already a medium BAB class geared towards combat.
