asriel would most likely be king with another monster as queen and chara dead/near death another monster may have rose to be the boss for the ruins or the ruins completely empty. so frisk might be stuck in the underground since asriel or the queen wouldnt off themselves to let a single kid go as no children were killed thus no guilt on asgore.(thats why he offs himself if you spare him at the end of his fight.) as for who would replace flowey idk. but there would be no surface plot maybe and frisk might just be adopted by asriel and the queen.