In XenForo 1 there are few classes that cannot be extended. Recently I ran into inability to extend XenForo_Template_FileHandler and XenForo_Template_Public. Those limitations make it impossible to write add-ons that modify templates at compile time rather than run time, which might affect performance if modifications require a lot of parsing. Changing it for XF 1 would require too many core changes, so probably not worth doing it now. Can you please make all classes extendable in XenForo 2.0, so it would be possible to change pretty much any part of XenForo?