A brave Sekiro: Shadows Die Twice modder has ported Elden Ring's Malenia straight into the former game, and it works beautifully.
The fight can be seen just below, in which Malenia has been transported into the area of the Divine Dragon boss fight in Sekiro, so the two foes can duke it out among clouds of atmospheric mist. The whole fight flows fantastically well, and it's worth checking out the full video in the replies to the original clip.
Malenia's been ported into Sekiro with a little help from Genichiro Ashina if we're seeing things correctly. Elden Ring's boss still has the Sekiro boss's thrust attack, which you can see represented by the red Kanji above her head, where the player can stamp on her blade and cut out a vast chunk of her stamina meter, seen at the top of the screen.
The entire flight works surprisingly well, but there's a crucial reason Malenia fits so well in Sekiro: the vast majority of her attacks are sword-based. In FromSoftware games we're used to side-stepping attacks, but Sekiro wants players to face down attacks and get in rhythm to parry them away, deflecting sword attacks like Malenia's to build up stamina meters and render an opponent open for a killing blow.
Malenia's attacks might be fast and incredibly furious, but since 90% of them are sword-based, Sekiro can stand defiantly against them, unlike in Elden Ring where you'd have to dodge through them. We'd hazard a guess that putting Sekiro up against Malenia might even make the Elden Ring boss a tad easier, but who are we to say?
Here's hoping we get to say hello to Malenia again in the upcoming Elden Ring DLC.