That's a tough question. I play a 5e game, with mixed rules as desired, but not really by my intent; I joined a group after relocating, so I'm playing what they're running. It's hard to imagine what would bring me back to D&D again after all these years, as a preferred game as opposed to just one I'm playing because there's one out there. I've NEVER been a huge fan of too many of the D&Disms, and I've always been looking for the Holy Grail of some game that does what I want it to with the elements that I want until I basically designed one myself.
That said, I'm more a fan of older D&D than newer D&D, even though older D&D wasn't really my thing either. But I feel like the direction the game has been heading for some time with implied setting, tone and many of the elements and mechanics are getting even farther away from what I could accept than even old D&D is. I presume this is due to me not being the market WotC wants to target, and I presume (somewhat skeptically, but still) that they think that they can continue to sell a product that is more and more focused on some kind of cozycore super-powered magical animal people and call it D&D, so I have no hope whatsoever that official D&D will ever give me what I want. But if it were to, for some strange reason...
Even 1e and BECMI started feeling really overpowered and high fantasy for my taste. In every edition of D&D, it's my experience that the most fun, the easiest to run, and in general everyone's actual favorite place to play (even if they romanticize the idea of high level play) is low to low-mid levels. This suggests that a darker, grubbier, low fantasy just works better, mostly. Certainly that's what I'd want. I want a tone and feel that's more like Darkest Dungeons the video game, or something like that. Make the game more serious, more risky, more edgy again, but do it in a different way that what came before; make it really lean into the darker elements and make that an integral element of the tone.
I also know that power-gamers and character build optimizers are well treated by D&D, although I think Pathfinder is even more of what they want; therefore, D&D 6e should step away from that and offer strong support for interesting theater of the mind combat as the default; tactical board-game piece movement and gridded maps can be optional, but needs to really be optional.
Everyone romanticizes the long campaign, but the reality is that shorter, potentially linked campaigns is much more realistic. Also, 5e talked a lot about bounded accuracy in the lead-up to its release, but the end result doesn't seem like it delivers anything like it to me, nor does anyone even talk about that anymore. Heck, I had to look up ten year old blog posts on my blog to even remember what the phrase was. This should absolutely be a real design goal. And the higher levels; do they even really need to be in the game at all? I prefer not to have them. I once played in a 3e campaign (Age of Worms) that went from 1st to 23 or 24th level, but honestly; after about level 6-7 or so, actually running the characters was just tedious, and combat was very long and mostly kind of boring. I'm not convinced from my experience now that 5e is different. Something like E6 that offered unlimited progression without actually giving us the tedium of high level play; true bounded accuracy; that would be great.
So there you have it. If D&D 6e were designed to my specific parameters, which let me be clear about, I think would be financial suicide, so I don't recommend it at all, but if it were, that's what I'd want. A combination of Call of Cthulhu and rules-lite OSR-like games, low magic, edgy and horror-like grubby low fantasy. The X-files or Supernatural the CW show in a fantasy setting.
