Elemental is probably the best bet if you feel the need to specialize. I did Ele, and I regret it. Not that there is anything better to specialize in for a rogue, but because its so freaking expensive, and VERY hard to farm for.
Keep in mind, I'm not compalining, and I think that the harder-to-get materials are necessary for such good gear, but I hate buying things on the AH, and the famring is quite tedious so I've all but given up Ele working. Just a friendly warning to take into account! ^_^
I actually really like the Wicked Armor set. Regular leatherworking and some very nice agility stats.