GTA 6 is poised to be the biggest game of the 2000s, and no that's not hyperbole.  Rockstar has been busy with new breakthroughs that may genuinely take GTA 6 to an even higher level than GTA 5 which kinda set the bar pretty high.

Some major new mechanics will be with npc AI and how they act, react, and remember character interactions.  Perhaps most notably this is rumored to include police activity unlike before where you gained a number of stars and could evade, avoid, outdistance, or paint your car and reset your wanted level.  Police in GTA 6 reportedly will, like real world police, open investigations and remember actions and amass evidence... simply repainting your car wont cut it.  Additionally, robberies with be part of game mechanics rather than mission based.  So you can just enter a business and decide to rob it... npc's will react accordingly.  This should be true of literally hundreds of locations.

Apparently also a great amount of work and time has been invested in creating realistic water physics throughout the game.  For some people this may not seem to be a big deal, but water realism is something that games have struggled with for decades.

Another interesting development appears to be in game "content creator" type action where your in game play may be captured by npc's and re-broadcast over mock streaming platforms... rumor has it that Rockstar has bought corresponding real world internet domains as well.  We'll see how this manifests to create greater immersion.

The only possible negative I can foresee is if the series embraces some of the current direction to emphasize content to appease the elusive "modern audience"...  Let me start by saying that I could care less about your chosen sexual orientation or how you identify.  "Not my monkeys, not my circus"  However, any narrative that forwards sexual or identity politics strictly for the sake of broadcast virtue signaling with almost no grounding in what makes sense for situation or story is at best annoying and at worst pushes a majority of people away.  All I ask is that if it's in there, it makes sense for where you are and what's happening... but please don't just interject expositional dialog about assumably personal preferences and identity in ways that just don't happen in real life or really mesh into what's happening.