The seven countries are not specified in the Trump EO, never mentioned in fact, what is specified is the existing list of nations where suspicion of terror infiltration may originate, as described by President Obama. The Trump EO simply defines when and specifics about the added scrutiny to be applied over the next few months.
So, the selection was made by President Obama, with good reason, and Trump is starting with this group of Nations as it is a good place to begin. Will further adjustments, added Nations make sense to address in addition to these seven? Perhaps, and those Nations may be addressed at a time decided, by legislation or by order as may be necessary.