I have a tickling suspicion that if any free-software OS has any chance at replicating that sort of global unification/consistency across apps, it's Haiku. Sure, there are plenty of apps that are just straight ports from the Linux ecosystem, but there are also lots of apps written first and foremost for BeOS/Haiku (or ported to Haiku exceptionally well) and consequently feel like they "belong" just as nicely as the ones that come preinstalled.