Wishlist: Make it possible to provide both - the OpenGL and the OpenGL2 renderer
In order to be able to provide software that uses the old OpenGL renderer and software that uses the new OpenGL2 renderer in Linux distributions it would be an advantage if both options could be enabled at the same time.
The alternative to install an older version of VTK to provide the old OpenGL interface has the disadvantage that this older version is not maintained and will not receive bug fixes.
Since both interfaces share part of the class names and header files*, I'd suggest to put the old interface in a separate namespace and install the according header files into a subdirectory, so that porting old software to the legacy code path can be done quite easily.
*) Personally, I think that this was an unfortunate decision, because as a result of this that when searching on the internet for VTK documentation one never knows whether the documented class is the OpenGL version or the OpenGL2 version, a grave problem given that the interfaces of equally names classes actually differ in some cases.