I think he needs at least one Kairoseki sword, or to have the tips of his blades lined with Kairoseki, so that they can cut DF users.
He already can cut most DF users anyway. What he can't cut are logia and Buggy. It would be lame for him to cut logia because his sword has some some cheap trick up it's ass. What if his sword happened to be stolen/lost? All of a sudden he is useless infront of a logia. Look at Shanks, (probably) one of the 2 best swordsmen in the world. He uses some ordinary (looking) sabre, not some gimmick seastone one.
Would have to be some wierd one like a heat DF (that isn't fire, just one that makes the blade glow in heat).
If he wanted that he would have allowed Usopp to upgrade his swords (not to mention a cheap rip-off copy of Sanji's Diable Jambe). What people don't realise is that giving Zoro's sword a DF is basically the same (and just as stupid) as giving Zoro one. E.g. if his sword were to have a logia fruit and he could use it's power, then the only difference between him and a real logia user is that he can't turn into the element and he can swim. And if were a zoan then all his 1 on 1 fights would become 2 on 1. That woud just cheapen Zoro's ability as a swordsman, and most of his victories would be credited to his sword and it's power, not him and his hard work and skill. Like I said before, if he were to loose that sword then suddenly he looses power and is weaker; he becomes a shadow of what he was with the sword.
Seriously, if it's ok/cool for Zoro to get a DF and/or seastone sword(s), then why not give Sanji one seastone shoe and one logia DF shoe and have him fight AoKiji? All he has to do is step on AoKiji's foot to stop his powers and then let his other shoe use some uber, deus ex machina type attack to OHKO AoKiji; then we all can start celebrating and discussing Sanji's development into such a powerful, world class fighter.