I've been thinking about reactive rewards for all ESO expansion maps, since ESO is adding various reactive achievements in the game, and I think it would be worth adding new things to get people excited about older content. All of this is inspired by what we're currently receiving, so I'd like to list them here:

Morrowind -> By completing all quests, exploration, fishing, etc., you receive a guar! (A simple guar with Tribunal art on the saddle)

Summerset -> By completing all quests, exploration, fishing, etc., you receive an Indrik! (An Indrik with the Alinor symbol on the saddle)

Elsweyr -> By completing both the stories and other achievements for both Elsweyr zones, you receive a Senche-Raht (or regular tiger)! (A Senche-Raht with Khajiit symbols from Rimmen, Riverhold, or the Dragonguard)

Greymoor -> By completing all the stories and other achievements for the Dark Heart of Skyrim, you receive a bear or wolf with the Solitude symbol.

Blackwood -> By completing all the stories and other achievements for Gates of Oblivion, you receive a Welwa with the Blackwood symbol on the saddle.

High Isle -> By completing all the stories and other achievements for Legacy of the Bretons, you receive a jousting horse or an ornaug.

Necrom - xxxxx already has the mount.

Gold Road - xxxxx already has the mount.

By completing each of the base game maps, the player could earn a pet representing the map:

Auridon with a cute yellow bird
Grahtwood with a small monkey
Greenshade with a tiger cub
Malabal Tor with a Maormer serpent
Reaper's March with a cat

MOUNT FOR COMPLETING THE DOMINION STORYLINE: Queen's Eye Indrik - An Indrik with the AD symbol, inspired by Ayrenn's new attire in High Isle.
PET FOR COMPLETING ALL DOMINION EXPLORATION ACHIEVEMENTS: A golden eagle.


Stonefalls - A bantam guar as a pet
Deshaan - A small Netch
Shadowfen - A lizard
Eastmarch - A sabre cat cub
The Rift - A pocket mammoth

MOUNT FOR COMPLETING THE PACT STORYLINE: A Companion wolf with the Pact symbol.
PET FOR COMPLETING ALL PACT EXPLORATION ACHIEVEMENTS: An imp that transforms into a red dragon.

Glenumbra: The dog that accompanies us throughout the main quest of Glenumbra (he literally gives the quest in Daggerfall and follows us to the end of the map. I never understood why he doesn't become our pet at the end). — If this skin is already used in a pet, change the quest dog’s skin to a new puppy.
Stormhaven: A mini dreugh
Rivenspire: A bat to fit with all the vampire-related content!
Alik'r: How about a mini duneripper?
Bangkorai: A raven from Uela.

MOUNT FOR COMPLETING THE DAGGERFALL STORYLINE: A lion with blue armor bearing the Covenant symbol.
PET FOR COMPLETING ALL DAGGERFALL EXPLORATION ACHIEVEMENTS: A smaller lion, similar to the larger one.

Personally, I think adding these rewards for already-released content would be very interesting, especially for retaining new players! What do you think? How would you change it?