Fontane was working on several projects when he died. As mantioned in Cahir Mawr Dyffryn æp Ceallach's answer, one of these was Die Likedeeler. "Likedeeler" was the Low German name of the "Vitalienbrüder" (Victual Brothers, a group of late 13th-century privateers). One of the most famous names associated with them was Klaus Störtebeker.
The fragments or drafts for Die Likedeeler were published for the first time in Hermann Fricke's study Theodor Fontanes letzter Romanentwurf: Die Likedeeler (Verlag der Rathenower Zeitungsdruckerei) in 1938, using manuscripts available at the Theodor-Fontane-Archiv. (The Theodor-Fontane-Archiv had been established just three years earlier in Potsdam in 1935.)
Fricke wrote that there were 240 manuscript pages in folio or quarto size, part of which had text on both sides, and 6 single-sided manuscript pages with notes about trips to Emden and Bremen.
The scholar noted that the manuscript pages had gone through many hands, so their original order had not been preserved.
The fragments were published again in a larger volume entitled Fragmente und frühe Erzählungen, Nachträge (Nymphenburger Verlagshandlung) in 1975 and in Fragmente. Band 1: Texte (De Gruyter) in 2016, edited by Christine Hehle and Hanna Delf von Wolzogen and published on behalf of the Theodor-Fontane-Archiv.
The 2016 edition is now the authoritative edition of Fontane's unfinished works: it
contains approximately 170 unfinished stories and essays by Fontane that were not published during his lifetime and which only exist as drafts. Approximately 90 of the texts are being published here for the very first time.
The fragments of Die Likedeeler can be found in the section "Historische Erzählungen" ("historical stories/narratives"). With a length of 58 or 59 pages, it is the longest of the eight narratives of that section in the book and one of the longest one in the entire book (Eleonore is just 5 pages shorter; Allerlei Glück is 70 pages long).
Since most fragments in the 2016 edition are at best a handful of pages long, Die Likedeeler is among those projects that had advanced furthest at the time of Fontane's death.
In mid July 1880, Fontane came up with the idea to write Frisian novels with the titels [Die] Likedeeler and Quade Foelke (see Roland Berbig's Theodor Fontane Chronik (De Gruyter, 2010), who uses Fricke as a source).
Eight years later, more specifically on 8 May 1888, Fontane had finished his draft for Frau Jenny Treibel but had not yet started working on Die Likedeeler:
With this story [Frau Jenny Treibel] I conclude the cycle of my Berlin novels, which amount to 6 in total, and, if a few more years are granted me, I intend to finish with a very ballad-like historical novels that is situated around the year 1400.
We know that Fontane wanted to include poems into the novel. Roland Berbig's Theodor Fontane Chronik notes for the year 1893 (my translation):
Plan/outline ("Entwurf") for Likedeeler — F especially gives thoughts avout the poems he wants to include in Likedeeler; titles for the envisaged titles are "Storm on St. Vincent", "Storm on Bergen or Drontheim", "A Song that includes their 'agenda'" and "Then a song celebrating storm and sea".
Berbig's entry for 16 March 1895 tells us that Fontane wanted to base the novel on the most exact historical knowledge; he asked both Hanz Hertz and the historian Friedrich Holtze for help in researching the novel's historical background.
On the same day, Fontane also writes (my translation):
I want to work on a new novel (whether it will be completed does not matter), a splendid novel that deviates from everything I have written until now and that deviates from everything that has been written so far, even though some will probably tend to put it in the same category as "Ekkehart" and "Die Ahnen". But it will deviate from these by being a reconciliation between my earliest and most romantic balad style and mmy most recent and most realistic novelist scribblings.
In Zum Geschichtsdenken Theodor Fontanes und Thomas Manns, oder, Geschichtskritik in "Der Stechlin" und "Doktor Faustus" (Königshausen & Neumann, 2004), Birger Solheim, using he above passage, writes (my translation; italics from the German text):
The plans for Die Likedeeler suggest in what direction the old Fontane was going. He emphasises that he does not plan to write a typical historical novel like Ekkehard or Die Ahnen: "It [Die Likedeeler] deviates from it, in that it should be a reconciliation between my earliest and most romantic ballad style and my most modern and realist novelistic scribblings" (IX: 25). So Fontane definitely had a conception of how a historical should be modelled from a narratological point of view, namely like a ballad with an ethically oriented hero and, as far as Die Likedeeler is concerned, with a hero oriented towards socialism.
(Die Ahnen was a cycle of historical novels in sechs volumes that Gustav Freytag published in the years 1872–1880. Ekkehard was a historical novel—based on the life of the monk Ekkehard II—by Joseph Victor von Scheffel that was published in 1855.)
Hanz Hertz writes in a letter to Fontane dated 3 April 1895 that he is busy with books related to Die Likedeeler and with a song about Störtebeker.
In spite of these plans, Fontane first focused on other works, such as the novel Der Stechlin (written between 1895 and 1897 and published in book format in October 1898), his autobiographical Von Zwanzig bis Dreißig (published in 1898) and the novel Mathilde Möhring, which remained "unfinished" at the time of his death but was published posthumously in 1906 .
According to Fricke, Fontane first started working on a Likedeeler novella but got stuck fairly soon. His original outline for the chapters turned out to be insufficient when he started addding historical information.
In a second attempt, Fontane drafted both an opening and a closing chapter, but the remaining chapters still raised many questions. Fricke assumes this second attempt still lies before 4 April 1995.
The third, fourth and fifth bundles of manuscript pages seems to represent a phase in which Fontane had access to more historical sources. Fontane also wrote the first version of chapters two, three, four, seven and eight.
Fontane's main source was Johannes Voigt's treatise "Die Vitalienbrüder", published in Historisches Taschenbuch. Neue Folge, zweiter Jahrgang, edited by Friedrich von Raumer (Leipzig: F. A. Brockhaus, 1841). In addition, he also used book about the history of East Frisia and about Klaus Störtebeker. Fontane also visited East Frisia in August 1882. (As mentioned above, Die Likedeeler was not the only novel related to East Frisia that he wanted to write.)
 Based on WolrdCat search results, this book is probably hard to find outside university libraries.
 The 2016 edition of Fontane's fragments and drafts contains a fragment of 2–3 pages for Quade Foelke. This would have been a novel inspired by the Frisian noble Foelke Kampana or Foelke the Cruel, who lived from 1355 – c. 1418 and was a contemporary of the Likedeeler.
 I write "unfinished" in quotation marks because Fontane had already offered both Mathilde Möhring and Frau Jenny Treibel for publication to two periodicals in July 1891, even though he didn't finish the first draft of Mathilde Möhring until late August or early September of the same year. Fontane reworked the novel in the winter of 1895–1896, after finishing Von Zwanzig bis Dreißig.
- Berbig, Roland: Theodor Fontane Chronik. De Gruyter, 2010. (3905 pages. See snippet view on Google Books.)
- Fontane, Theodor: Fragmente und frühe Erzählungen, Nachträge. Nymphenburger Verlagshandlung, 1975.
- Fontane, Theodor: *Fragmente. Band 1: Texte. Eeited by Christine Hehle and Hanna Delf von Wolzogen. De Gruyter, 2016.
- Fricke, Hermann: Theodor Fontanes letzter Romanentwurf: Die Likedeeler. Verlag der Rathenower Zeitungsdruckerei, 1938. (See snippet view on Google Books.)
- Solheim, Birger: Zum Geschichtsdenken Theodor Fontanes und Thomas Manns, oder, Geschichtskritik in "Der Stechlin" und "Doktor Faustus". Königshausen & Neumann, 2004.