Riley has stated previously that the compendium is something they are keen to expand to other systems and even to include game/account specific 'custom' compendium information. However, this will only ever be likely to occur once the 5e compendium has been fully implemented and tested to breaking point to ensure that the system is robust enough to cope with other systems. It basically allows them to focus on getting one 100% right before adding more and potentially having to trouble-shoot several systems compendiums all at once! (please note that some of this is paraphrased based on the information I've seen Riley himself talk about in the forums on this subject and in no way reflects the road-map as it exists for the Roll20 development team!) In short, it'll come in time... till then we just have to wait and dream of all the things we can do with such a tool (I know i am!) :)
