There are several options.
The current head could in agreement with the remaining dynasts and the collateral branches of the Saxon house opt for a semi-salic or gender neutral succession.
Another option would leave it to the next branch or adopt a male dynast from another branch.
Maybe he's lucky and his daughter Leonie falls in love and married the younger son of Andreas of Coburg.
That might provide Michael with a few grandsons to continue his line.
It was reported the death of Pr. Georg-Constantin of Saxe-Weimar-Eisenach after a horse fall at an equestrian event http://royalmusingsblogspotcom.blogspot.com/2018/06/prince-georg-constantin-of-saxe-weimar.html http://www.noblesseetroyautes.com/deces-prince-georg-constantin-de-saxe-weimar-eisenhach-1977-2018/
Prince Georg-Constantin was the last dynast of the Saxe-Weimar family and had been appointed by the current House Chef, Pr.Michael as his sucessor.
Prince Michael only had one daughter, but the salic laws rule her out of the succession.
Pr.Georg-Constantin’s father is still alive but aged 72 and currently divorced.
In 2015, Pr.Georg-Constantine married Olivia Page but they did not have any children.
So another branch of the Ernestine Wettins is facing extinction, after Saxe-Altenburg.
The Saxe-Meiningens are also in the verge of extinction as the current Duke is unmarried and has no children.
Maybe he could adopt his step-nephew Friederich-Constantin from the morganatic 1st marriage of Friederich Conrad’s father Bernhard IV and delay the extinction for another generation.
Friederich-Constantin is 38, also unmarried but has one illegitimate son, unable to succeed unless his father marries his mother (?)
With the extinction of these 3 lines from the Ernestine branch, the only one that would remain is Saxe-Coburg-Gotha.
In the older days, the remaining lines got together and “divided the spoils” of the dying line, namely its properties and wealth, and the titles.
After Sx-Altenburg and in a near future Sx-Weimar and Sx-Meiningen extinction, would it be feasable for the then Duke of Saxe-Coburg-Gotha to claim the heritage of this 2 branches and become a sort of Grand-Duke of one unified Thuringia ?
I asked feasable, not probable, as the Sx-C-G are rather low-key.
I read that the Ernestine succession was ruled by the Pact of Erfurt of 1640 and the treaty of Hildburghausen of 1826 but could not trace them .
When Sx-Altenburg died the other branches just let it go down or did they shere the “spoils” ? Properties, Orders, etc. ?
It's too soon to speak of Sx-Weimar, but is it known if Friederich-Konrad of Sx-Meiningen has taken any provisions for the future of his house when he passes out ?