If NewJeans was meant to be a 7 member gg, (as they were apparently originally were) the only two other members I could add would be Aran and Iroha.
Aran would be a lead vocalist and Iroha the main dancer. I think both suit the group very well. Even age wise too 
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Aran bright's, friendly and lovely energy on top of her jazzy, feel good, relaxing vocals would be a perfect addition to the NJZ's in my opinion.
Iroha on the other side would blend in smoothly in the teams visual and dance line. Her sense of hip hop is INSANE and would be a good addiction to the team . Her natural youtfull charms would be perfect.
Also NJZ having a japanese member would be a nice addiction. She would just have to take vocal lessons to keep up with the girls, but maybe she could also develop potential rap skills? Her gentle and kind personality would blend in well in njz, and her high professionalism is remarkble in my opinion!
Following that,njz's danceline would be: Iroha, Haerin, Hanni, freaking killer line!
And vocal line: Aran, Hanni, Dani, + Hyein!
Of course I prefer NJZ ot5 which i find to be a PERFECT team, but the addition specially of somone like Aran would be so AMAZING in my opinion. Her voice sounds so perfect for NJ's songs and i feel like it would blend so perfectly within njz's vocal line and specially with Hyein jazzy tone
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And I added Iroha because I don't like 6 member groups, and I would have liked a japanese member too, looking at it, visual, age, personality, stage presence, talent vise Iroha was the best choice 
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ps: The added members have to be between 2001/2009 liners.
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. MUCH better...
