I just got this thought going through my head right now--anyone remember The Broken Steel DLС that changed the Fallout 3 ending completely? When your alter-ego or none of your friends didn't have to die in the end, you could instead ask Fawkes the supermutant to push the button, and everyone was alive and happy? I believe that dlc also gave an opportunity to free roam after the main quest and end titles. Before that expansion, you couldn't do that.
So I thought that it would be great if CDPR would take a look at that DLC and take some notes. That would actually help in establishing the one and only canon ending by introducing some changes in the existing ones to bring them to one universal outcome, or even introduce a completely new ending and call it canon. It may be a happy ending or a bitter ending. It's up to them. Honestly, I don't know why this idea never crosses other developers' minds: to introduce a DLC that changes the original story a bit and not just adds a couple of hours of new story and gameplay.
Because for now, all of the endings vary completely, and in case the devs would like to make an expansion after the main story, they'll have to pick one, and thus disappoint those players who picked the other one.
It's really weird, imo, to make a DLC at least Hearts of Stone length (10 hours or so) when V is literally coughing with blood and can barely stand on his/her feet BEFORE the last Nocturne mission. Not to mention that such DLC would be as long as half of the original story, which is around 20-25 hours or so.
Storming the Arasaka tower solo with Johnny and then becoming the legend of the Afterlife seems to me like a perfect epilogue tbh. The most epic and thrilling ending out there.