I dunno, Abercrombie is an amazing author and I haven't enjoyed anything as much First Law since the first few reads of LOTR but I just don't think it'll be a good match.
Although there are plenty of similarities between the two, what makes FL a particularly good read is it's humour. The witty, lighter dialogue would need stripping back and the prose to be a far more formal tone. You wouldn't find Martin writing 'six hours later he was drunker than shit' unless it was said by a character. You take that away from Abercrombie and he loses a bit of his magic.
You're probably right. However, I think that his writing became a bit better after TFL (standaloes and the Shattered Sea have a better and more formal writing) and also there isn't as much humor in Red Country and The Shattered Sea compared to TFL and the first two standalones (unfortunately though, the books aren't as good as those 5, which makes you probably right).
If Abercrombie cuts some humor, and goes a bit more formal (like he has proven that can do) he's the closest to Martin right there. He has shown that he can handle big stories with a lot of interesting characters, can write good politics, in addition to being very good at writing. I don't see any other (from those I have read) that have these characteristics. Also to be fair, Martin has a lot of dark humor though.
For example:
Abraham: can handle big stories but cannot handle more than 2-3 characters at the same time. Writing is decent but not great. Politics suck.
Rothfuss: cannot write more than a book for a decade, hasn't proven that he can handle big stories. Writing is excellent though, even better than Martin's. No politics.
Sanderson: struggles with characters, stories centered around magic, writing has gone from barely readable to very good, but still not on Martin's level. Politics suck big.
Hobb: old, over simplistic politics, not too many characters in her stories. Writing is excellent though.
GGK: best writing ever in the genre, but don't go for sequels. Hasn't shown to write stories with a lot of characters, though his characters might be closest to Martin's. Also old.
Lawrence: see Abercrombie, remove some points.
Lynch: a less magic-based of Sanderson, more interesting characters but like Sanderson, he hasn't many characters.
Gaiman: old and shit, the sub genre is very different to others.
Bakker: assuming that he can spend only 50% of time writing philosophy on his books, then we might have a good match. But we know that Bakker cannot do it, so please no.
Erikson: haven't read, might be a good match.
Weeks: haven't read, but from the description seems like Lynch.
Cook: good match but he's even older than Martin.
Brooks: old and apparently shit
Goodkind: see Brooks
I think that the list covers all big/promising authors in the genre, and I do not see any better match than Abercrombie. He also has been open to the idea.