This is a quite a complicated what if Mike.

In strictly genealogical terms the title would presumably have transferred to the descendants of John Frederick I, the dispossessed cousin of John George’s grand-uncle Maurice. Let’s say that John George I died in 1612 (i.e. a year after his accession and year before the birth of his son) then the descendant of John Frederick I at that time would have been John Frederick’s great-grandson the 15-year-old John Philip, Duke of Saxe-Altenburg. The dispossession of John Philip's great-grandfather and his own youth and near-dispossession by his uncle John II, Duke of Saxe-Weimar might have caused difficulties on a legal level however.
  
The following family tree gives a good overview of the dynasty:
 
House of Wettin Family Tree

(It falls down a bit in the detail, however, by giving dates of accession only rather than dates of birth as well. This can sometimes be misleading because younger offspring are sometimes located to the left of older siblings rather than conventionally on the right.)