What seems likely at this point is that the Ernestine Wettins will eventually dwindle to one single surviving branch: the Saxe-Coburgs are the only Ernestine branch that show some promise of lasting into the next generation or so. The territorial divisions within the Ernestine Wettins were never understood to be permanent, and when one line became extinct the family as a whole would have to agree on a new territorial division between the surviving branches - but always whilst maintaining succession (to both lands and titles) within the male line only. The last such occurrence was in 1826, when the extinction of the Saxe-Gotha und Altenburg line necessitated a fresh division of territories by agreement between the surviving lines, under the arbitration of the King of Saxony as head of the Albertine branch.
This is what makes Prince Michael's current position rather delicate: he and the heads of the other surviving Ernestine lines objected to the late Margrave of Meissen's dynastic innovations because they violated prior usage, and because they failed to obtain the consent of the House of Wettin at large. But now, facing the imminent extinction of his own line, he is apparently trying to designate his own daughter as his dynastic successor, replicating the same solution that he declined to accept for the Albertine branch. I would be surprised if this "solution" garners much sympathy or acceptance from the princes of the Saxe-Meiningen or Saxe-Coburg lines.
Regarding the Albertine Wettins: can we really even say that the Saxe-Gessaphes have "inherited" anything? The legal issue hinges, I gather, on whether the late Margrave of Meissen actually changed the house laws regarding dynastic equality, or whether he simply declared his nephew to be his dynastic heir by fiat. Everything I've read on the issue up to this point suggests that he followed the latter course.
As Noel points out, what would have happened is that the territories would have been redistributed between the other surviving reigning branches. The only point of contention might have been the Grand Ducal title which should perhaps go to the male primogeniture line. On the extinction of the entire Ernestine male line either the succession would pass to the nearest female to the last male, or a "Baden" solution mighe be found, by which the nearest morganatic male line is demorganaticised. But this could not be done when not reigning as it would require the act of a reigning sovereign - and the chances of restoration are slim, to say the least. However the Saxe-Meiningen line has a lot of Freiherren von Saafeld, descendants of Ernst, brother of the last reigning Duke. When the latter abdicated, Ernst refused the succession and had already married unequally. Of course there are plenty of Saxe-Coburgs - aside from the senior line headed by the representative of the Duke of Albany, excluded from the SCG succession by a morganatic marriage that was actually to a noble woman (whereas the present Duke of SCG is married to a commoner), there is of course another line, considered dynastic, and which retains the right to SCG should the junior lines become extinct - that is represented by HRH the Duke of Gloucester. The renunciation by Edward, Prince of Wales was made at Windsor Castle on 19 April 1863, and was accepted by the Landtag of the Duchy and then on behalf of the three younger British Princes by their legal guardian, King Leopold of the Belgians, at Laeken Castle a month later. This renunciation retained a contingent right of succession for
himself and his issue in the event of the extinction of the male line descendants of his younger brothers. Therefore, should the male line of Duke Andreas become extinct, then arguably the heir would be whoever is Duke of Gloucester.
The Saxony branches don't really have many eligible male dynasts.
Meiningen only has Friedrich Constantin at 37 still unmarried.
Andreas of Coburg has two sons and one grandson. His daughter-in-law is 43 so unlikely to have more children and his younger son unmarried at 41.
Saxony itself had to go to the Gesaphe branch through a female line.