I've actually considered the premise several times (That is: A map wherein the teams attack/defend actual corporate headquarters or possibly even privation mansions housing blutarch/redmond) but the wall I keep coming up against is that getting that far into either teams base sort of brings the tf2 storyline too close to a finality, whereas the way it's written now the battle could literally rage forever. I realize that no one is talking about ACTUALLY making blutarch/redmond killable, but still, a home or office invasion type objective would almost force valve to continue the storyline somehow if the map gained enough popularity, which might go against something they already have in the works.
Just my .02
I think there's good ways to incorporate actual office/mansion/not-your-usual-industrial-desert-tf2-map-thingy without forcing it through the storyline like that.