please for the love of god
Which god? Is it heresy I smell? The repurgators would like a word with you...
Ok seriously now. First Larian would need to get a deal with Games Workshop. GW is really careful about its intellectual property and may impose a lot of restrictions, many more than Wizards of the Coast apparently. This would really limit Larian's freedom of work.
Second, the Warhammer Fantasy world (at least the world before the End Times, not the Age of Sigmar nonsense) is a pretty dark and desperate place. Larian has some great storytellers, but would they enjoy and perform so well in a grimdark setting?