i know this might not be popular with some people but here is something they can do

Setting:
Valenwood and Sommerset Isle

Time:
within 30 years of the events in skyrim

Plot:
Main quest:
the discovery of a new race of beast
fish like npc
two different kinds:
normal: like snow elves (you can comunicate with them) very powerful mages, have underwater or a floating city (where you can visit and trade with them and have some quests for them)
savage: like falmer (attack everything) it is a disease that spreads specifically to this race and the inventor of the virus can control them to do his/her bidding. this would be the main villian you have to overcome because he is using them to try and invade the mainland (huge army of savages). the story ends with you killing the leader and they just start to wonder around aimlessly attacking people or TES could decide to find a cure for them.

Civil War:
after the civil war in skyrim the altmeri dominion loses its control over tamriel

the empire rises to fight back against the thalmor with the help of their allies

The Altmeri Dominion:
Altmer
Bosmer(the vast majority others are rebelling)
Dunmer(the Altmer enslaved them as punishment for not joining them and are forcing them to fight against their will)
Orsimer(fight for the Dominion in return for glory and land)

The Empire:
imperials
Nords (the Empire has agreed to drop all claims to Skyrim and will let them rule themselves in return for their aid)
Redguards(the empire agreed to drop all claims to Hammerfell and will let them rule themselves)
Bosmer(the few who are rebelling against the Thalmor)
Dunmer(those who escaped the enslavement of their race)

Neutral:
Argonians
Bretons
Kajiits

Story for Civil war:
the empire and its allies have driven the Thalmor all the way back to Valenwood and have a foot hold in the top 1/3 of Valenwood (this is where you would recieve your quests for the empire)

The Empire Quests:
the gist of it is that you thwart the Thalmors plan and inspire an uprising amongst the Dunmer slaves which will weaken the Thalmor and you eventually drive them out of Valenwood (they still control Sommerset) and with that the Bosmer cut ties with the Thalmor

The Altmeri Dominion Quests:
you crush the Bosmer rebels and then drive the Empire out of Valenwood

Travel:
boats will be a key feature to this and add options to where you can board other ships(like robbing a merchant ship)
you can get your own boat and upgrade it until you have one that is equivalent to a galleon(warship)
can get a crew for your ship(companions who can be a boarding party and serve other functions as well)
the Savages will randomly try to board your ship (they are fish like so you can encounter them anywhere on the ocean)

Factions:
Mercenary Guild- like the fighters guild
a lot of land based missions however some might be that you have to protect a merchant ship from pirates (enemies with pirates)
Pirates- this will replace the thieves guild for this area
missions will be to rob ships as well as obtain several priceless artifacts on land (enemy to the mercenary guild)
Dark Brotherhood- yes its back and stronger than ever
after the events of skyrim a new listener was named and he has brought the Dark Brotherhood to its former glory
some missions you might have to sneak on a boat and kill someone and sneak back off without being noticed
Psijic Order- a very mysterious and very powerful group of mages(the are mentioned in the College of Winterhold Quests)
in this time of great uncertainty they come out of hiding

Magic:

new spells!
water spells like wave that would send a wall of water at somebody (knocking weaker enemies down) and a spell that when cast on an enemy would make a water bubble form around his head causing x-amount of damage per second for a certain amount of time among other spells that they can come up with
earth spells like earth spike where spikes come up out of the ground and impale and enemy causing severe damage, chunk rock just a spell that would hurl a boulder at an enemy, earth wall defensive spell that makes a wall appear infront of you to protect you rock armor i think that this one is self explanatory, these are just some earth based ones that they can do
wind spells: gust hurls enemy backwards(like fus roh da), some kind of temporary flight one, there are several others that you could make with this

spells interact with environment
ex. cast water wave while on the ocean and a huge wave will be created capable of knocking enemies off the deck of their ship
ex. cast gust at your sails to get a temporary boost to your speed

new Armor:
some kind of mage robes that are useful
some kind of nature oriented armor for the Bosmer(light armor)-better than elven armor but not as good as glass
aquatic looking armor from the fish race (light armor and heavy armor- best armor can only get after completion of main quest)
only craftable you can buy so you would need perfect smithing skill)(takes the place of the dragon armor)
dwarven: not new just not in this game
redguard armor: don't know what it should look like but it replaces the dwarven armor as far as its armor rating

Weapons:
thrown weapons like fire bottles, knives, stars,etc
spear
hammer
Claymoure
Battleaxe
Waraxe
dagger
mace
sword-long and short one handed
staff-magical staffs that can be used as a melee weapon at close quarters
bow
crossbow
blowgun(can fire silent poison darts or darts that will knock somebody out)

combat:
nonlethal:
new stealth actions that will allow you to knock somebody out(sneak up behind them and hit them with the hilt of your weapon)
attacks with fists or shield will drain opponents fatigue eventually knocking them unconscious
lethal:
enemies on horse back
can use spear to knock enemies off their horses(or impale them off if their health is low)

opening scene:
you are a prisoner on a Thalmor warship and you are being sentenced to death for being "an imperial spy" a Justicar takes you back to your cell to await execution and as he is putting you into your cell the player hears screams and shouting from above deck. your cellmate(a bosmer rebel overpowers the Justicar and tell you to follow him to the lifeboats. as you leave the brig you see the body of another Justicar collapse and you get your first glimpse of a Savage. you have to fight your way off the ship and you get to the life boat to escape. while paddling away from the war ship you see Savages climbing up the sides of the Warship. then a Savage climbs up onto your life boat and knocks you out(you see the bosmer rebel kill it as your vision blackens). you wake up in a bosmer rebel camp and your savior tells you to go see the Imperials at their camp in northern Valenwood. this is where you are now able to free roam and do whatever you want. you also get that life boat as your first boat

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