lol well... I believe you're mostly right Vega. There are some skills that would be specific to a Dark Knight and some skills you could equip on your Lancer (and advanced jobs associated) for a penalty. Yes, very similar to the way it is now. There are Gladiator skills that would work best for DRK's and PLD's (Gladiator is no longer playable, it levels with your DRK and PLD) that can be equipped on any job, equivalent to the way it is now for everyone else using a different job's skills. DRK and PLD each have their own specific weapon type that defines them, and until they are unlocked, equipping that weapon just makes your job GLA.
Kuro's original post suggests Dark Knight and Paladin being essentially the same class with a different role (comparing DRK to a PLD/NIN and then a PLD to a PLD/WAR). In that sense, your job is still tanking/damage mitigation, and with a Talent Point system you could change your specialized class very easily. Since DRK and PLD are so similar, there really is no need to level both jobs independently. Don't think of it as a DRK from FFXI, which is a magic + high attack DD that has much lower DEF. Think of it as a PLD with some DD boosts. One class (Gladiator) with two different ways to play it.
Ayerc and I have been discussing a slightly different method. Within a base class (such as Gladiator), give it two specializations (Dark Knight and Paladin, much the same as Kuro's) but make them much more specialized than Kuro's. In this sense, you can think of them more like FFXI jobs. A Dark Knight wouldn't be nearly as suitable for tanking as a Paladin. But they are similar in the sense that they both possess some magical abilities (Light and Dark) and have moves that enhance their respective professions, so they would share common abilities and traits from GLA, which would still level up with them (similar to physical level). Because the jobs are so different, instead of a Talent Point system where you could switch between the two, keeping the same level, you'd need to level both independently (in order to "learn" your job, and get the full experience, etc).
Besides that, everything else is the same with Kuro's post. I've been kinda waiting for Kuro to get back to see what he (she?) thinks about all this, too, because the majority stems off of his (her?) ideas, and I'm curious to know if and how he (she?) might implement them into his (her) plan.