One thing when modeling that To Be, remember a pertinent version of the 80-20 rule; spend 80% getting the usual process, and 20% on variants. Working as a consultant, I almost always only get one pass at this, so do it all at once whenever you can.

And if you are focusing on requirements, gettiing them from the relevant business people whose time is always limited, I wouldn't worry about gap analysis, that will happen when your designers compare the TO BE requirements against whatever the current system is; the Requirements are the same no matter what.

Dave