There's no substitute, don't search for it
Destiny 2 is basically an umbrella, you CAN be the type of player that enjoyed some niche part, be it its activities, or becoming a collector, the lore expert or the raid nights and at the same time find a game that fills that niche.
But if you loved Destiny as a whole, I'm sorry, but there's no substitute for it. I don't mean it as doomposting, see it as a friend trying to help you, truly.
I tried Division 2 and Warframe, I played somewhat a good bit of Warframe but dropped after feeling locked behind needing platinum to unlock more Warframe spaces. The gameplay didn't do it for me, but the gathering of resources, crafting and acitivity diversity was what appealed to me. As for The Division 2, I just feel the game is bland. It does its homework well and I think that's it. There's no appeal or hook, it's just a fine game. I think I would like it more if it was a FPS, but being third person really puts me off.
This is just to say that, before the inevitable end, I tried searching for it and got to this conclusion. The only thing that may come close to a substitute is where you'll find friends and build memories again. When you find a special place to call a second or third home. If you go to these games with the refugee mindset, you'll get yourself disappointed. They're good games, just not what you want. You want Destiny, we all do.
Mourn this loss. Play the update. Try to gather your friends, like I am trying, for a last hurrah. You can even make news friends that are in the same emotional state as you. Do some final moments with Pantheon Nights or SRL fun. Anything, really. Take your time.
When the time comes, just like Destiny in 2014, you'll boot a game without even knowing that it may change your next 10 years forever. You'll find home.
Eyes Up, Guardians