I have found an excellent ressource on the territorial evolution of the Ernestine Duchies, aka the bulk of modern Thuringia (
link here, it's a collection of digital maps made by the University of Mainz), and that way I could contextualize some stuff on the Treaty of Division at Hildburghausen, the treaty which divided the lands of the recently ended line of Sachsen-Gotha-Altenburg.
I will use the map of the Duchy of Sachsen-Gotha-Altenburg in 1820 to demonstrate this.
The original porposal, made by the second-to-last duke of Sachsen-Gotha-Altenburg in 1821, shortly before his death, was the following:
1) Sachsen-Meinigen should receive the territories which are administered from Gotha [which at the very least includes all the exclaves west of the one that includes Kranichfeld; I don't know the status of the Treppendorf and Ämmelstädt ones]. This would also probably also include the Amt Römhild, which was up to that point a condominium of Sachsen-Gotha-Altenburg and Sachsen-Meiningen.
2) In return Sachsen-Meiningen will cede their exclave of the Meininger Oberland (the modern county of Sonneberg) to Sachsen-Coburg-Saalfeld. This exclave is the territory between the two large chunks of Sachsen-Coburg-Saalfeld.
3) Sachsen-Coburg-Saalfeld would also gain the lands of Sachsen-Hilburgshausen (I found no mention of the future of the exclave between Römhild, Bavaria, and Meiningen), but in exchange the duke of Sachsen-Hilburgshausen would gain the Altenburg lands.
This proposal was rejected due to Sachsen-Meiningen claiming that they would become the senior line and that they should inherit all lands belonging to Sachsen-Gotha-Altenburg without any other territorial changes.
In 1825, when the line of Sachsen-Gotha-Altenburg actually went extinct, Sachsen-Meiningen also made a new proposal different than what was adopted in 1826 (
s. here), which was to have Sachsen-Meinigen to become a contiguous North Franconian duchy by annexing the lands of Sachsen-Coburg-Saalfeld as well as the Amt Römhild and the Duchy of Sachsen-Hildburghausen, and in exchange allowing the dukes of Coburg-Saalfeld and Hildburghausen to take over Gotha and Altenburg.
EDIT: The wording of the 1825 proposal of a North Franconian duchy made no mention of Saalfeld, only Meiningen's claim on Coburg, so in theory Sachsen-Coburg-Saalfeld might become Sachsen-Gotha-Saalfeld instead of just Sachsen-Gotha.