cryengine3 is an automatic open world sandbox editor and can make the world open for you. I know where you are coming from and what kinda gameplay you have in mind coz i also have the same desire to make open world games. If you wanna use cryengine3 pls do so to make it possible for you to achieve your dream but even though there are good sides to cryengine, there are also the bad sides. The bad sides are that until now the crydev ( cryengine ) is still a baby in providing you or anyone else with tutorials and info about the depth of the engine, but i am sure as years pass by the community will develop and grow so that anyone can reach out for help and gain knowledge by just searching and practicing them out, but for now it is a difficult task to do so. I myself used the cryengine as soon as it came out, but came towards many blocks and i couldnt search for any help for all those small needs, But then i quickly changed my engine and chose to use the unreal engine 3. Why?, well first of all the unreal engine ( UDK ) has a vast amount of resource for the community and people of interest like you and me and even though it there is no info out there which teaches us how to make an mmo open world game ( which is strange ), i also agree with you that there are developers out there who has made great MMO RPG games using UDK, so guess its a secret. I believe that one day, some one will put a tutorial about making a open world Game with UDK and this will solve everyones problem and even if no one puts it out there for us to read, i am sure that one day UDK will update and suddenly it will have kismats or something else which will allow us to easily create a MMO type game just like what cryengine is doing right now. So my suggestion and recommendation is to study the UDK and just hold on until some kind of resource about making MMO games will be found by you in the search engine or it will be given to you in a free update.