You should listen to our audio media posted lately. It answers this question well from the horses mouth.
There will not be "another mod" or addon. After MP is completely done in our goals for it, we will start to plan out a SP branch of GE:S if the team can agree to continue at the pace we are. Likely it wont be just replicating the original missions verbatem, but expanding its scope based on plenty of ideas we have.
We actually have 7+ maps still to produce before SP is a chance to start, and several animation discussions need to occur internally before that is agreed on as well. The code base is nicely in place to handle a SP branch by the time we get that going.
Fans following since 05 waited this long to get this solid and awesome experience from MP, we would hope those hanging out for SP had similar patience, its very tough to work for years in spare time, its a big sacrifice you must understand.
