I know nothing about programming but I think the only way another altcoin could become successful is to use a different hashing algorithm other than scrypt or SHA.  Perhaps an implementation of bcrypt or SHA-3 could be used?  This would bring a whole new level of security to the cryptocurrency system.  Litecoin is strong because any weakness in the bitcoin network isn't shared by the litecoin network.  All others alts are clones of one of these two, so they don't actually add any sustainable value.