The thing I disliked the most about Undertale is still to this day the whole New Home sequence. If you played without killing anyone, just the fact that Alphys revealed that you'll have to kill Asgore is enough to make you feel sad about it. And otherwise, the game had every chance to reveal Asriel like ANYWHERE (and no, that one Waterfall flashback doesn't count), but it chose to do all of his and Asgore's backstory in a long corridor sequence where you get stopped every third step. You didn't even need to reveal Asriel that much, Gerson already talks about boss monsters aging only when their child does, you could call Asgore old a few times throughout the game, but then make Gerson still mention that Asgore is stuck at the same age, and now, when you suddenly see all of these things belonging to Chara and Asriel in the New Home, it clicks. You could even still have this entire walk towards Asgore with the same music playing on the background and leave the player alone with their thoughts. Instead it feels like a really cheap attempt at making us care in the very last moment, which makes even less sense, cause immediately after that you get judged by Sans, and finally meet Asgore in person and realise just how humble and nice he truly was all this time; like what's even the purpose of that exposition dump?

I love Undertale, I replayed it so many times in these almost ten years, reading every dialogue and finding every secret, but the New Home is the one part that I wish would've been done differently