Posted this comment on another thread and was asked to make it into its own thread.
Feel free to add your own thoughts/observations and opinions!
Disclaimer: They’re all different people and individuals, none of them are exact matches and this is just for fun.
Nayoung is a pretty good fit. She’s the leader, oldest and has more of a mature vibe. They’re among the better members at girl crush and sexy concepts and even tho they look very different both have amazing proportions.
Sakura= Sejeong and Pinky/Kyulkyung
Sejeong and Sakura are the second center of the group. They have their own brand of charisma and even tho they’re quite different they excell at the same things. They know how to sell themselves and come off as very professional.
Kyulkyung and Sakura are the two most charismatic visuals and also foreigners.
So most people would argue Sohye just because the two of them are the least talented in their respective groups but except for that they don’t have that much in common. I personally would say Chaeyeon is a better fit. Part of the visual-line and has a “pure” concept. Initially comes off as shy but is very sweet, kind and friendly. And even tho Chaeyeon has never been called a talent hole she’s probably the second weakest ioi member.
All rounders and variety show aces. Born to be idols and have the the most “swag” in the group. Easy pick!
Chaeyeon= Chungha
A pretty basic combination but it works. Dancing queens and the two most talented members. They’re both pretty capable leaders to boost.
Chaewon= ...Sohye
Unexpected but for some reason the two of them remind me of each other. I suppose they’re also one the cuter members.
Yeonjung and Yuri are the main vocals of IOI and Izone. Yoojung and Yuri are lookalikes but not very similar except for that.
Hitomi= Mina
Cutie dancer with squishy cheeks. 
Nako= Mina?? Yoojung?
Cute dancers again. Mina is a weaker vocal but has a cute tone just like Nako. Yoojung is below 155cm (she pretends to be taller) so guess she and Nako could bond over being tiny.
Minju= Pinky/Kyulkyung/Jieqiong and Chaeyeon
Part of the classic visual duo. Minju and Kyulkyung are both labelled as the “main” visual of the group (tho Chaeyeon got more attention for her visuals in the second half of ioi’s career, especially in Korea). They’re the visual member most often used for girl crush stages and they’re more charismatic and talented than Hyewon/Chaeyeon. Both rocked pink hair! In many ways Kyulkyung=Minju is pretty perfect but in other ways Minju really reminds me more of Chaeyeon. Kyulkyung is loud! IOI as a whole was crazier than Izone and Kyulkyung was one of the wildest members. She was a very energetic and fun performer and has always been better at girl crush concepts than cute ones. Minju can be hilarious too and has gotten real good at maturer concepts but it didn’t come as natural to her. On Produce she had a girl next door vibe much more similar to Chaeyeon. Talent vise Minju and Chaeyeon are also more evenly matched.
Somi and Yujin’s personality’s are pretty darn similar. Half the time I’m seeing Yujin it feels like I’m watching Somi. But if you focus more on their function in the group Doyeon might be a better choice. They’re more or less as talented as each other and get similar lines. Plus they’re both tall and part of the maknae-line.
Wonyoung= Somi and Doyeon
So Somi and Wonyoung are both the center of the group, the giant maknae and often considered the 4th member of the visual-line. But as I said above Yujin’s personality is more similar to Somi’s. Doyeon is part of the maknae-line as well. She’s the tallest member and a visual. She and Wonyoung could be a pair.
