Source of the information
All the information found in this post is based on the three articles that were released recently on the official KR community website for Goddess of Victory: NIKKE:
Game overview and team composition tips - link,
Story mode overview - link,
Materials and shops overview - link.
And what's important - all the information is based on the official release, not the CBT or Technical Test.
Since the blog post will be quite long, we added a TL;DR at the end.
The blog post was edited on 28.10.2022 to remove parts that were also removed from the original sources.
Video version
If you prefer to watch a video version of this blog post, check this YouTube video.
Game overview
Let's start with the UI. The main screen hadn't changed much compared to CBT2 and TT, but one thing caught our eye instantly - take a look at the top-right section of the screen:

Mission Pass widget can be found there, together with what looks like a casual skin for Diesel - the free character that will be given to all as a pre-register reward.
So while the article itself doesn't mention it, we just got a confirmation of a Battle Pass and Skins not only existing in the game but also being available on the release! While both aren't anything new when it comes to gacha games, it's the first time we can safely say NIKKE will get both.
The rest of the article goes over things we already knew:
there's no stamina system, so you can play as much as you want - until you hit a wall that will force you to idle and wait for resources,
after the tutorial, you will unlock the Auto Battle system which has pretty decent AI, but you will still have to use manual when you will hit the wall and will want to tryhard to push forward,
weapons have different ranges and you have to create a balanced team that covers all ranges or you will be forced to constantly switch characters,
Shotguns dominate in the short range and only Sniper Rifles have no penalties against enemies in the long-range,
Assault Rifles are the jack of all, master of none in the game.
elemental system exists, but it isn't as important as the Burst system.

If you want to learn more about weapons and team building, do check our guides as we already went over both: Team building guide and Weapons guide.
Story mode overview
The second article explains the different battle types you will encounter while progressing through the story.

There are three main battle types and one special type.
Annihilation
Annihilation is the most common battle type where you simply have to kill all enemies to pass the stage.
Long War

In this mode, enemies will keep moving from left to right and you have to stop them. Each time an enemy reaches the goal line, you will lose HP points based on the type of enemy:
normal monsters are worth 1 HP,
elite monsters are worth 3 HP,
boss monsters are worth 5 HP.
You start with 20 HP, so it isn't much and you have to be quick with killing whatever appears on the screen. Thankfully, as long as you have at least 1 HP when the timer hits 0, you will win.
Compared to Annihilation, this mode appears quite rarely in the Story Mode, but quite often in Side Stories.
Stronghold Defense

This mode is a combination of tug of war and capture the base modes from other games.
The timer is set only at 1 minute and you start with 50/50 split in points between you and the enemy. You obtain points by keeping your base clear of enemies, but for each enemy present in the base, they gain points instead and you lose them.
To win the battle, you have to have more points than the enemy once the timer hits zero, but if your points reach 0, you automatically lose.
Seems pretty simple, but there are a few important factors:
Flying enemies don't give points even if they sit atop the base so you have to focus on ground enemies first,
Auto Battle is kinda annoying here as often it focuses on the flying enemies first leaving the ground ones in the base,
Shotgun characters are super strong in this mode as not only there is a lot of enemies clustered together, but also they often stay in the short range.
Compared to Annihilation, this mode appears quite rarely in the Story Mode, but quite often in Side Stories.
EX Stages

EX stages are basically a mode within a mode. Instead of a single stage, you enter a map where you have to manually lead your team from battle to battle, till you find the boss and kill it.
Among the four modes, EX Stages are the hardest.
Materials and shops overview
The first part of this article focuses on the materials used to upgrade characters, but we already have a guide that explains it in detail: Character progress guide.
Let's move to the various shops available in the release version. There will be 4 of them, but the number doesn't include the Cash shop.
General Shop

In the general store, you can buy a lot of items both for Credits and Crystals (premium currency). You can get Credits for free daily from here, so don't forget to check the shop when you log in for the first time each day.
Union Shop

The first big surprise in this article confirms the existence of a guild system which is called a Union in NIKKE. In this shop, you can buy boxes with resources needed to upgrade characters and the cube.
The article also reveals that the Union system already will have some additional modes - on top of allowing the players to gather and conquer the game together. The two modes locked behind the Union are:
Shooting Range - in this mode you have to kill as many enemies as you can and you are given a score at the end.
Raid - nothing is known about this mode yet as it wasn't open during the tests the author of the article performed.
Body Shop

In this shop, you will be able to buy things using currency obtained after max limit breaking a character in the Recycle Room (more about this will be revealed in a different article).
Mileage Shop

In this shop, you can buy SSR characters for Silver Mileage Tickets that you obtain after pulling - 1 pull = 1 ticket.
Currently, there are only 6 characters available in the shop: Privaty, Crow, Aria, Eunwha, Mary, and Ludmilla.
Gacha Tickets
The third article also reveals the existence of two tickets.

The one on the left is called Golden Mileage Ticket and the one on the right is called Advanced Recruitment Ticket.
Sadly, besides the description, not much is known about those two tickets as you could not obtain either of them yet. Still, the ART looks like the typical selector ticket that allows you to pick one character available on the ticket - so we can safely assume it will be locked behind a paywall.
TL;DR
Battle Pass has been confirmed to be available on release,
Skins are confirmed to exist in the game - and some will be tied to the Battle Pass (not sure if in the free or paid track yet),
Guilds - called Unions in NIKKE - with Raids will be available on release.