All slots I think would be bad, but theres no question JC needs much loving. I thought about doing it once. For about 40 whole seconds. Then I added up certain costs, looked at what sold etc. This took about 5 of those seconds, because like 4 things sell. Black sapphire earrings are a big one. Earring of choice for casters pre-raid environment as far as I'm concerned, based on low cost/benefit. For at least one slot if not both. But I've always said "man they should have more kinda epiclike pinnacle items for jc, b/c the earrings are great, but they just well...kinda suck comparatively." Oh and the first person who says they aren't epics because I know that's going to happen....I realize that. Thats why I said like. Like saying the pinnacle of your craft, but less wordy.
Perhaps for a slot increase=> Wrists. People do wear jewelry on their wrists. Soooo where is the jewelcrafting for it? It makes sense, and its not like, oh, here's my ruby encrusted cape of stuff, or man, my diamond boots are pretty sharp and uncomfortable. Crowns are another possibility that makes a certain amount of sense. Since crowns are pretty much just fancy hat jewelry.
As far as the shadowsilk/imphide/deepmetal argument goes insofar as jc is concerned, I think b.s. might need a boost tbh to compete. Like its been said before, most people do not want diamondine. It's prohibitive cost, and most people just not giving 2 craps about resists enough to sacrifice that amount of stats is probably the reason. It comes down to a maybe if you're a monk. But I know quite a few monks who still don't bother with it. Diamondine could get a boost too, and that might be a more acceptable idea, to leave b.s. as they are for the pre-raid crowd, as they aren't too powerful for that, and well placed. Perhaps the addition of a new type of gem. Something rare like Painite (oh man it could come from torment

). Just pop up a set of stuff based on that, along the lines of deepmetal/ssilk, add in perhaps a highranked crown from a couple of rare drops, and like a wrist, or earrings or w/e. Then wham, there ya go.
One of the places that definitely should get targeted insofar as profitability is concerned is definitely the pre-raiding to like maybe torment tier. Above that, well soon you won't have a need to buy much other than the big ticket items, which is one of the reasons jc needs at least one of those, but if you hit that market square you'll be at least garnering some profits from it.