I haven't seen any solid information whether the next iteration will be a refined Navi or their next generation. But if it turns out to be Navi 2x, as you mentioned, the scaling from 5700 to 5700 XT shows how little room there is, so a ~50% larger chip would have to be clocked lower than 5700 to retain decent efficiency. Whatever AMD launches in the Navi family, it will retain the same scaling characteristics. So for this reason, it matters if AMD launches another iteration of Navi or something which (hopefully) is a more efficient architecture. If AMD's lineup for 2020 consists of refreshed and possibly larger Navis, then it's not going to be good enough, which is why I have several times before stated that a second Navi in 2020 is just too late to the party.
Lastly, just to put it out there, a second iteration of Navi may very well be some sort of professional version too, but we have no details on that.