Classic example is the depicted Demon of Hatred from Sekiro. Don’t get me wrong - the fight does work in Sekiro. But it would feel at home in something like Bloodborne, at least that’s what most fans say.
Is there any other boss you’ve encountered where you thought they’d fit a different soulslike better?


Unpopular opinion here but the SEA Spider from AC6 should have been a souls boss. Not a lot of people remember that the armored core series is fromsoft, but it is. Something about the movements and attack pattern of the SEA Spider felt more like a big ungainly souls boss than it did an AC boss. The Smart Cleaner almost felt that way too but it still feels more like an AC boss.
I could imagine it as a boss form for Sellen. Rather than helping her or Jerren as soon as she gets to Raya Lucaria, first help her face off against a Carian knight to take over the academy. When you come back the first time, she’s a regular graven mass, but Rennala is still sidelined
In the courtyard after the Red Wolf of Radagon boss room, you make your choice for the quest. You can either help Jerren fight a fallingstar beast (called down by Sellen as she peers deeper into the primeval current) to side with him, or you can fight him yourself (in which case he gets some backup from a few Lucarian scholars who remained loyal to Rennala and Sellen casts magic artillery down from the tower to help you)
When you next enter the hall, either with Jerren to kill Sellen or by yourself to continue your apprenticeship, her graven mass form has spider legs made up of the same type of material as the fallingstar beasts. All of the sea spider’s missile and laser attacks get reflavoured as magic. The missile one that launches upwards is basically already the graven mass barrage in fast forward, and the laser is comet azur. I think it’d work great