Already existing endings are:

Pacifist - After sparing Noelle, Alex gives up her SOUL to the monsters. An aborted route also exists, where if you kill Noelle, you go on to fight the Neutral final boss.

Neutral - After getting judged by Luna in New Home, one of Alphys' DETERMINATION experiments attacks you. After the fight, the experiment resets. No, it's not Flowey.

Genocide/Rampage - I call this Rampage sometimes because Scratch is a kids' website and that's the coding engine I grew up with. The final boss is Luna, who can't take damage from your attacks due to being a ghost. However, one of the New Home enemies is a boss monster. Absorb their SOUL and defeat Luna using magic. After the fight, go to the castle and kill Asgore. Alphys' experiment tries to get in your way, but you kill it. After that, Perso, AKA the Integrity SOUL, appears on the screen and starts talking to Alex. Perso explains that they came back because something awoke them. Perso then goes on a long speech about how the world is no longer interesting, so they and Alex should erase the world. Regardless, the "no" button doesn't work and you have to erase the world. After coming back, the title screen appears as if nothing happened.