>>323201434
I suggest you don't look it up if you don't want spoilers, but it's quite something.
You need to do some specific heists that have you gather some special loot (golden grin casino being an example, if you remember that weird black/golden chest that's in the vault).
[spoiler]Once you do all those required heists, their loot is available for tinkering with in the safehouse. Tinkering with them + some secret stuff in correct order will progress the secret ending route. After that, in a quite interesting way you get a list of achievements which you must complete to continue progressing. The achievement list you must do is unique to each player. Once you AND 3 OTHER PLAYERS all do your achievement lists, the 4 of you can go do the final heist where you'll get to take the secret ending route. The rest is fucking hard, that's all I'm gonna say.[/spoiler]
I intentionally tried to be as vague as possible as I believe you might get a better "wow" factor from doing all those things yourself and not watching it all in a video like I did.
Honestly I'd be down to trying doing this as it can be done on Overkill and it's really interesting content, but it will require achievement farming and it seems it'll require some sweaty gameplay for the final heist.
Think I'm gonna start doing the story mode too when I can, wanted to get it over with for quite some time now.