I'm a firm believer that instead of Holly being taken in S5, it should've been Erica (or Mike).
With Erica, we could've explored the Sinclair sibling dynamic a lot more considering it barely happens in the tv show (compared to the white sibling duos e.g. will/jonathon, Mike/Nancy).
It would have also given them the opportunity to explore how Lucas feels about his little sister being dragged into the upside down mess. With the aforementioned sibling relationships, the younger siblings where the entire reason why the older ones were in that situation, so they can't really tell them to stay out of it when they're the centre of the issues (e.g. s2, will being possessed etc).
But with Lucas and Erica, he has full authority to keep his little sister safe, but was unable to and now he has to deal with the ramifications of this whilst dealing with his friends' issues as well. We never actually see him get mad at the scoops troop for dragging his little sister into a Russian base and risking her life, nor do we truly see how he felt on s4 seeing his friends decide to play DND with his sister instead of going to his game and supporting him.
This then gets worse in s5 when they're barely together, Lucas as a character is centered around Max and Erica is reduced to the sassy black woman stereotype with little to no screen time.
It would have also been really interesting to see proper discussions of race happen between the 2 characters, and how that plays into their story. It always annoyed me that people just brush over that interaction Erica had in s4 with Jason when he was looking for Lucas as her just being sassy. That would have been terrifying for her, especially considering the mental state Jason was in.
Plus, with Erica in Henry's mindscape, trapped in memories with Max, we coukd jave explored their dynamic too, something that is not done in the show despite Max literally dating her older brother. They could've given Erica a more complex character arc, showing her to be the one who's actually scared and puts on a brave face and issues she's surely also faced like Lucas did in the earlier seasons (Troy and Billy).
Overall, it just would've been nice to explore the sinclair siblings in way more depth instead of sidelining them for the rich middle class white family.
(I'll do another post as to why Mike would have also been a good replacement for Holly's character, if anyone would want that)