It’s also really weak that Palpatine tells Rey he wants her to kill him (so that he can possess her) because saying such a thing would actually de-motivate her to follow through on that plan. I just think the whole “kill me, and ascend the throne” thing is really dumb and could possibly be cut out. Palpatine just isn’t this stupid and ignorant.
As Hal said, v2 makes a couple changes to make his plan a little less jarring. We’ve made it so that Palpatine has always wanted her dead, and originally wanted to possess Kylo once he’s killed Rey, but when he turns back to the light is when Palps changes his plan to possessing Rey.
As for him explicitly saying that to her, I’d imagine it’s because there wasn’t really any other place in the film to show just what his new plan was. I think he’s kinda hinging on Rey’s naivety concerning essence transfer (ie she doesn’t know for sure if she will retain some level of autonomy). He’s trying to give her the impression that only by merging with him will she have enough power to save her friends. He says as much, “We will be one.” Of course, we know that it isn’t merging, that it’s replacement, but she doesn’t.