I haven't seen this mentioned by anyone to date, but doesn't it occur to everyone that the hammers we've seen pictured thus far bear some resemblance to weapons used by those in the pictures? For example, in the Herc/Juggernaut picture, the hammer looks like Herc's mace. As another example, the hammer in the Creel/Ms. Marvel/Giant Man picture has a chain, and thus evokes Creel's wrecking ball. Somehow, it seems, Thor's death/absence/defeat triggers weapons in the MU to transform themselves into character-specific knockoffs of Thor's hammer.