I wanted to compile a post that collects all the various quests contributed to TR as showcases, which never got merged into the mod. After surveying 10 pages of quest showcases, here's what I've collected:

Vvardenfell:

Fields of Kummu Thief - a quest makes you solve the problem of a muck-stealer at the Fields of Kummu shrine. Locations: Fields of Kummu, a muck farm nearby, and Balmora. Status: Quest is finished by Wannax and reviewed by DevShah. Has some minor unaddressed styllistic issues.

Robbing the Ancestors - a quest starts in Andothren with a MG apprentice asking you to rob an ancestral tomb on Vvarndenfell with him, so he could repay a debt to Camonna Tong. Status: quest is finished by FlinSunset and reviewed by Mort, but has few remaining styllistic issues. A bridge quest for mainland-Vvardenfell. Update: moved to Othmura.

A Dangerous Delivery (could use a more specific name) - it starts in Seyda Neen; talk to Valgus Cnisius on the upper floor of Arrille's Tradehouse. It also involves a visit to Balmora and an encounter along the road to Hla Oad. Status: successfully finished by CogAndStar and reviewed by Mort. Could be a nice addition to vanilla starting location.

An Unquiet Grave - west of Seyda Neen there's a new NPC who asks you to kill a zombie (a corprus stalker), who turns out to be his former business partner. The zombie is still sentient and gives you a counter proposal to avenge him. Status: complete by WellTemperedClavier and reviewed by Mort. Another neat addition to the starting location.

Necrom:

Nodachi for Anjzhirra (this shouldn't be the final name of the quest; I made it so the reviewers would have an idea of what's inside) - a Khajiit trader in Old Ebonheart sends you on a treasure hunt quest giving you a name of a Necrom fishermen. Said fishermen will point out a certain underwater treasure for you which, after wrangling it from under the guard of some dreugh, you can either sell to Anjzhirra or keep for yourself. Status: requires a modern review. Made by Steelgonads.

Old Ebonheart:

Scholars' Research Dispute - a quest about two rival scholars which will have you make a choice between a cruel elder scholar and his apprentice. It starts in Old Ebonheart, will lead you through either Necrom or Jobasha's store, then will make you visit College of Firewatch, and will terminate at Thirvayn Ancestral Tomb in NE Telvannis. Status: finished by pkk and reviewed by Mort. A good addition to the imperial scholarly institutions in mainland, which currently are on the lacking side of content. 

Memory Well - a quest adds an NPC behind the Old Ebonheart EEC building, who makes you dive into a new underground interior cell, that has to do with memory stones, which leads you to the Mages Guild. Status: the quest is finished by hardycore and reviewed by Mort. It has some dirty references and minor styllistic issues, but otherwise playable. Could be a good addition to OE city adventures.

Andothren:

Guar Sabotage - a noble in Gol Mok gives you an assignment to mess up the guar business of his rival at Llaram Plantation in Roth Roryn. Status: Jonado will release it as a mod. Authors: anathemastudio, Jonado.

Velothi Mountains:

Headless - the player encounters a living skull in Romavel Ancestral Tomb north west of Bodrum. He then asks them to find their living relative in Boethia Spine so he could be buried properly. Status: done, but has endless possibilities to be built up, the quest development continues. Author: Ph.D. in Sorcery. Reviewer: Mort. I haven't played it, but I recall that at least an MVP version of this quest could be merged into the mod already.

Hlan Oek:

A Missing Associate - two traveling merchants got cought in a vindictive Cammona Tong trap. The player decides whether to help them or benefit from their demise. Status: showcase ongoing, but the quest is playable. Author: _Artano_. Reviewer: Pralec.

Darvonis:

Simple Cartography - a FG member steals a map from a cartographer who's been staying at Darvonis inn, thinking, that it leads to treasures. The task is to track him in Sundered Scar and retrieve the map. Status: ready for merge. Author: Don. Reviewer: Pralec. The quest needs Sundered Scar finished.

Thirr Valley:

Vandirelyon Puzzle and Boss Fight - this quest makes you interact with unique items in Vandirelyon, Underground to open a door leading to the fire atronach boss fight. Status: Vandirelyon was redone from scratch so the future application of the quest is uncertain. Author: Boznirith. Reviewer: Why.

Ideas on What to Do With These:

1. Finished quest claims could obviously be reviewed and merged.
2. Abandoned showcases could be made into individual quest claims to be finished and merged.
3. Vvardenfell-only quests could be merged into a separate esp and uploaded as an optional download on TR's nexus page. Tamriel Rebuilt Vvardenfell Quest Pack - or something like that.

Very Old Submissions:

Aleft Investigation - this quest leads you to the Dwemer Ruins of Aleft near Gnaar Mok, where you could obtain upgradable automaton companions. Status: the quest is playable but have technical and styllistic issues. Author: Marrinara_Sauce. Reviewer: Rot.

A Quest in Balmora - don't really know what's going on in this quest, but it exists. Status: reviewed, but from 2018. Author: RyanG.

The Quest for Lute - another quest in Balmora from 2017 I didn't get to know the contents of. Status: reviewed by Kevaar. Author: Meej-Dar.

Cave with Slaves - a quest that has a cave that you can access in Balmora with bandits and freeable slaves. Not sure what's inside, but putting it here for future exploration, and to save time on finding every functional quest among the various showcase threads.