I don't know why but I'm glad there isn't the "terror" of sekiro in elden ring. Confront the beheaded and the Shichimen Warrior without suffering from terror It's very difficult.
I also wanted to see other bosses like Looking Glass Knight, Throne Watcher and Throne Defender, Burning Ivory King, Fume Knight etc. But unfortunately the ds2 graphics models don't allow it, you can do boss fights inside ds2, but transferring a boss from ds2 to elden ring is not possible as far as I know. If there is any mod that allows this it would be cool but I don't know any.