So you want Vice City's story taking place in Seattle? It's too similar to GTA:VC's story.
In Vice City's story, Tommy was released from prison after ten years and started building his own criminal empire and took over the Vice City.
Main theme of your idea is also A to Z, just like Vice City.
I will do you one better!
We go back to vice city and re-visit that exact principle!
In my idea Vice city would be a condensed version of miami, the everglades, key west, the carribean, cuba and colombia. Each region has its own police force, population and resources. In this way we introduce a couple new radical ideas. first and foremost it would be the first gta in multiple countries or in any country other than the U.S. Secondly it would provide completely different playing environments which would change your playing style. So if you want to not get hassled by cops you could be in cuba, or if you want to fight narco guerillas in columbia you can do that too. This as a map would provide so many terrain, foliage and city options as well as gameplay mechanics.
Lets go back to the idea of taking over the city. Remember in san andreas we had the gang warfare missions? Create 100's of these zones all over the map, these areas will be controlled by different gangs, factions, militias, warlords or politicians . each zone will have certain properties to it like income, crime, influence and enforcement. when you take over a zone you decide whether to have it make you money legally or illegally. legally would be buying businesses, houses etc, illegally would be deciding if you want to sell guns, drugs or prostitutes in the area. or you can do both .this will then effect the properties of the zone and add or lower crime, the income of the residents and the amount of police patrol in the area. This will change influence, which is the other half of this mechanic.
Political control.once you gain enough influence you are then able to sway politicians, drug lords, millionaires and others to help your cause. these will be your main missions, and will unlock business opportunities in your zones. so for instance you influence a drug lord to have you grow some drugs for him, you pick a zone that the drugs would grow well in and profit. this does however affect your influence on politicians and influential people. Influence will also allow residents to join your crew, in this way you will have security in your zones and be able to use them to take over other zones.
Now these mechanics will grow in scope through progression but essentially remain the same structure wise. and this can be showed in the story.
In my story you are a court marshalled marine, you had a brief psychotic incident that lead to the death of 2 of your squad members, at least that's what they convicted you for!
you have a different story, a troubling story of corruption and abuse of power. you knew too much and you got sent to Guantanamo bay. this is where the game starts, you literally see because a guard pulls the bag off your head! he covers your mouth with his hand but there is something in it that he shoves into your mouth. it says that he knows too and that he is getting you out of there. you start the game with a prison break from guantanamo bay and have to get by in cuba until you can get state side. Cuba is lawless or at least this version is, no police just people looting shooting and rioting! you learn the basic mechanics of the game here. here you will take over your first zones which will give you access to a document maker who will give you a fake name and id. another couple zones and you get a boat ride to america. once in america you contact your cousin, a low level street hustler in miami, who then introduces you to the gang he is in. the adventure takes off from there.
The story runs off the principle of domination you want to take over as many zones as you can and maximize profits in each zone. somethings are profitable in certain zones while others are not. for instance prositutes in the middle of the jungle wont make any money but drugs will. your choices of legally or illegally making your money will determine how the game plays out. you will choose your own morality whether to build a great empire to remember your fallen soldiers or to burn everything and reap the benefits.
capturing zones is a game in itself each one may have to be captured in a different way. you may have to kill a warlord with a hundred troops or bitch slap a shop owner and tell him to leave. most will be a good enough challenge to make them a reward when acquired and some will have multiple parts. they will also need to be defended, you can universally control the weapons all your crews use the cars they drive, body armor and other upgrades that make them more capable of holding off an attack, crews will gain experience in each battle won making them more and more effective.
this is just the core of the idea, as far as vehicles, guns and characters go its already 12:30 and I need to go to bed.
let me know if you like it