Not Really a re-release

According to Bethseda it has

Enhanced Gameplay

The gameplay in Quake II has been shined to a polish to improve the feel of the game while keeping the original spirit intact. Weapon fire from players and enemies now have muzzle flashes to improve feedback and new damage indicators to better communicate to players where they are taking damage from. An increased tick rate ensures action is more responsive and hundreds of level and gameplay bugs have been fixed.

Upgraded AI

The forces of Stroggos are deadlier than ever with improved pathing, new attacks and new tactics. Enemies will hit the dirt to dodge shots and jump down off ledges to get a better angle, making enemy encounters more dynamic. Flying AI has been rebuilt to allow for better and more realistic navigation through the air. Watch out for the Berzerker’s new leap attack, restored from the game’s original development, and dodge Parasite tongue lashings which will now stick into the wall, temporarily stunning them.

Improved Lighting, Shadows & Models

Levels have been enhanced with improved lighting, along with updated weapon and enemy models. Dynamic shadows and lighting have been added to levels and weapon effects. New glow maps make computer terminals, power cores and other illuminated surfaces pop. These graphics settings and more can be customized through the in-game menus.

Includes Both Original Expansion Packs

This release of Quake II includes the original expansion packs, The Reckoning, and Ground Zero. All the AI enhancements from Ground Zero are now available in all episodes. Additionally, for the first time, Quake II 64 has been ported over, making this exclusive episode available in high resolution and high framerate with modern controls.

Brand New Episode: Call of the Machine

In the depths of Strogg space lies the Machine, a singularity capable of collapsing the fabric of reality. Fight across time and space to find the Strogg-Maker, destroy it and change humanity's destiny in the brand-new episode, Call of the Machine. Created by MachineGames, this follow-up to their Quake episode Dimension of the Machine is now available for both single player or in cooperative play.

Navigate With The Compass

Navigate your way through the vast hub-based worlds of Stroggos with a brand-new compass item that will direct you to your next objective, no matter where you are, with useful arrows dropped into the world.

Open the id Vault

For the first time, dive into the id Vault and explore concept art and gameplay footage of Quake II, including early versions of weapons, items and even playable levels from the game's original development.

Split-Screen and Online Multiplayer

Quake II includes 4-player split-screen local multiplayer on all platforms, with 8 player split-screen available on Xbox Series X|S and PC. Fight against your friends in Deathmatch, Team Deathmatch and Capture the Flag, or team up in Cooperative games available for all five campaigns. Fill out your slots with AI bots for both Deathmatch and Team Deathmatch modes.
For the genuine QuakeCon experience, LAN play is also supported across all platforms and Nintendo Switch players can frag locally through ad-hoc wireless multiplayer. Mix up your games with custom settings such as Instagib and random weapons to maximize your fun.

Crossplay

Play with friends no matter which platform they prefer. Crossplay is available on Game Pass, PC (with controller), Xbox One, Xbox Series X|S, PlayStation 4, PlayStation 5 and Nintendo Switch versions of the game, so you can play Quake II with everyone. Join your friends quickly using our room code system for finding lobbies.

Enhanced Cooperative Gameplay

Save games are now supported while playing cooperatively and new rules have been added to improve Quake II's Cooperative modes. If all players die, it's game over and the level will restart. Respawn your teammates by clearing all enemies in the area or finding a safe place to hide. Teammates will spawn near you, letting you get back into the fight quickly.
Create an additional challenge for you and your friends by setting a maximum lives limit or upping enemies' health. To help with communication, you can also ping areas in the map by using the “point” gesture that will be visible to all other players.

AI Bot Opponents

Quake II bots can be added to Deathmatch and Team Deathmatch modes in both offline and online matches. They are more aware of their surroundings, including danger, and can perform advanced maneuvers like rocket jumps. With six skill levels from Training to Nightmare, Quake II bots are fun for both beginner and experienced players.

Plus they also added accesibility features