I think they should reintroduce the Firmir (under a different name), as a race of underground nasties.��The warhammer equivalent of Tyranids.��They could burrow their way around the world in the same way as skaven.
Grimstonefire
While anything is possible and while I think fimir are indeed cool as well. I don't see it happening. With the limited probability of CDs there is no way they will release a fringe model range. Comes down to resources. By the time GW possibly makes a new CD army book were talking late 2009. If that happens then CDs become one of the first 8th edition books rather then the last 7th.., again idea is cool, just not likely.
i dont think they will dump 7th ed in 3 or 4 years of running it. i think 8th is a long way off.
Malificant
One thing I've learned about GW: never underestimate them, especially now since they are run by a bunch of suits now. Love Warhammer great game, GW... not so much.
i think your best bet for a fimir army is using counts as chaos mortals.
as far as i can remember from hero quest they are servants of the chaos warriors pretty much, in warhammer all i know is that they live on swamps in albion.
using chaos mortals will let you have warriors and sorcerers, and then if you want to make up some kind of chariots and cavalry and things.
judging by this picture it looks like the easiest way of converting them would be from lizardmen saurus, as they have the same basic body shape, and an existing tail.