So...I've put this game on the backburner since its release. I have a character made and saved and I went through all the tutorial stuff. I'm just waiting for this addon to come out.

How much of the original game will this addon improve on?

What all are we getting with this that makes the game better?

If you first played this at release and haven't touched it since a lot has changed even not accounting for the 2.0 patch.
The upcoming changes aren't tied to Phantom Liberty either, they're part of a content patch for everyone that's dropping on the 21st. CDPR has actually released an overview of what's changing in general and what's part of CP:PL.

So if you want to just check it out after sitting on it for 2 years just get the patch on Thursday - it's definitely a lot more playable and less buggy than on day 1.
It's probably best to start a new character if you hadn't played that far anyways, and since almost all of the Phantom Liberty content is for higher level characters anyways there's still 100+ hours of gameplay you can milk out of this one before deciding if you want to spend money on the DLC.

Cyberpunk 2077 patch 2.0 gameplay changes:

Redesigned skill trees and perks
Revamped Cyberware and new capacity system
Vehicle combat and care chaces
Combat AI improvements
New police system
UI and UX improvements
Loot, items, and crafting changes
New radio stations

Cyberpunk 2077 Phantom Liberty gameplay changes

Dogtown – a dangerous new district
New storyline and characters
New quests, gigs, boss fights, and more
Vehicle missions and airdrops
New Relic skill tree and abilities
100+ new items, weapons, cars, and clothes
Vehicle missile launchers
Level cap increased to 60