Shouldn't it be legal since both the Elder Scrolls series and Fallout 3 are owned by the same company?
It'd be nice, but that's not the case. Each game uses different licensed content, different legal bindings, etc.
IMO, the best thing to happen to Oblivion was Morroblivion, but it was beaten and stabbed to critical condition (or death, I haven't checked in awhile) because it was a tool that could be used to convert the original files in Morrowind (which had to be present, thus owned by the person) into Oblivion. It was still deemed illegal by the Beth legal staff.
edit - there were some Pagoda models created for Oblivion weren't there? Those might suite your purpose, and looked to bepPlayer made to me (and not edited from Oblivion).