Even if Lego wanted to produce a D&D licensed line, I suspect that it's highly unlikely that they'd be able to obtain that license. Wizards of the Coast already produces an extensive
line of miniatures for use in D&D games. If they were to grant a license to Lego, they effectively duplicate their own product, and that's not good. So it's unlikely that Lego would be able to procure the license.
I also don't think that D&D is a good fit for Lego. Lego is, first and foremost, a child's toy; D&D is for an older age group. While I recognize that some lines are geared towards older builders, this is based on the sets being more complicated, and not because they are more 'mature.' If a child has the ability to build something from the Ultimate Collector's Series, for instance, then there's nothing in the content that might give the child's parents reason to pause. A D&D theme, however, is likely to give many parents reason to pause. Is this too 'scary' for my child? I don't want to make any judgements on the validity of this sort of worry, but it would cause some parents to worry. And, Lego has been very careful to avoid even the appearance of this sort of worry (a theme built around prohibition rum-runners aside
So, I've argued that Lego would not be a good fit for Wizards of the Coast, and I've argued that D&D would not be a good fit for Lego. Would I still want to see a D&D theme despite this? This is far more personal, but for me the answer is no. While it might be great fun to have Lego made D&D miniatures, I think it would be even more fun to try to adapt what Lego has already made in order to make my own. This is, for me, the great fun of Lego, trying to make something new out of a limited set of components. I recognize that this does not hold true for all.