It is something I would like to see
WoTC do as well (mainly since be in Character Builder and Compendium). But Setting Builder books. So have 3rd party companies make books showcasing mechanics, plot hooks, etc. for
4e that take it into different settings such as Steampunk, Pseudoscience, Horror, Dieselpunk, etc.
Really that is overall my one really big want for books in general for
4e (PHB3 is getting Psionics my other big want). Plus firearms would be something that could be done in the Setting Builder books.
Oh and a setting book for
Bas-Lag for
4e. I know Adamant Entertainment is making a roleplaying game for it. But I don't think they have released any info on what system they are using (or their own), but if they made it for
4e I would buy it in a instant (though I still plan on buying it anyways :P)