Source: https://www.reddit.com/u/headin2sound/

TL;DR: This is not a DLC. This is Cyberpunk 2.0.

virtually every system of the main game has been changed and/or updated
police system is completely reworked with multiple tiers of NCPD/mercs chasing you down in vehicles according to your wanted level, apparently on level 5 maxtac will chase you down and it will be some sort of bossfight against them (source: https://www.youtube.com/watch?v=riK1i8lQwRM&t=511s)
perks and skills have been completely overhauled, no longer simple passive stat boosts, much more active abilities like different melee finishers, a dash, or the ability to deflect bullets with melee weapons
an entirely new 6th skill tree that uses a different type of skill points (relic points) that will be used to add new abilities to your cyberware
the difficulty curve is reworked/rebalanced
the loot tiers are reworked
archetypes of enemies have been redone for more variety in combat encounters
installing cyberware now has an effect on your body and you cant install too much or else you will go cyberpsycho (no idea what that looks like in game)
installing cyberware now has a first person cutscene added to it, just like in the prologue at Vik's clinic
you can now "attune" cyberware to one of your attributes, making it more efficient if you have invested in that attribute; example here optical camo is attuned to the "cool" attribute: https://i.imgur.com/1UI9SEX.png
armor is no longer tied to clothing, it is instead tied to your cybernetics
vehicle combat is added to the game, you can even use your katana on your bike
you will be able to hack vehicles, similar to the Watch Dogs games
vehicles are no longer bought from fixers, but bought through a website set up by wakako
some vehicles have guns installed on them (machine guns, rocket launchers)
new type of infinitely replayable/repeatable missions are being added where you need to steal certain vehicles marked on your map
new activity introduced "airdrops", loot caches that drop randomly on the map that you can fight over and retrieve for yourself
tons of new random events/activities are added to make the world feel more alive, like car chases and gang fights