I am an American fan i think SM needs a powerful girl group with members who can rap so great, dance so great and also composer their own music and be so damn amazing on stage, not just sing, A multi talented girl group with attractives visuals and skills on stage. A Powerful SuperM female'll be:
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Yoona: Face of Group, Center and Leader of group, Main Rapper, Lead Dancer, Lead Vocal. Composer.

Krystal: Visual of Group, Lead Dancer, Lead Rapper, Lead Vocal.

Wendy: Main Vocalist, Composer.
Hyoyeon: Main Dancer, Main Rapper, Main DJ
Irene: Lead Rapper, Lead Vocal, Lead Dancer, Composer.

and also Lina: Main Vocalist (* Lina is the grace's member, Lina is the Best female vocal from 1st gen and even in SM's kpop history)

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Bonus:

+ Yoona is #1 Most searched kpop solo Artist on baidu 2019 (*baidu the most popular, most powerful and the Largest search engine in China and All Asia.

+ Irene and Yoona are two of the most popular sm female idol in korea.

+ Yoona was chosen the most popular SM female solo artist by U.S. kpop fans 2016-2018
+ Yoona is the SM female solo artist with the Biggest fanbase in Russia 2019 (russia the largest country in the world by area).

+ Yoona and Wendy are 2 of the smartest kpop artists.

+ when Yoona was still in high school, during an inter high-school competition she was chosen as one of the best young composers by experts.

+ Yoona did handwrite 100 letters with poem style to one of her friends who was going to another school and city

+ Hyoyeon is a great DJ.

If SM really want a successful girl group in the world not only in Korea and in these times, then they will need to choose SM artists so MULTI-TALENTED that they can RAP, sing, DANCE, COMPOSE MUSIC, be POWERFUL on the STAGE and be MULTILINGUAL besides being very beautiful, sexy with special charms. sm must give them so cool music with interesting and artistics lyrics and powerful dance choreo, The western market music is very difficult, but not impossible.

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Therefore thats powerful girl group will be:

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Yoona

Tiffany (sm should hire her again)

Krystal

Hyoyeon

Yuri

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They are even very popular in the Western especially Yoona and Tiffany. Yoona is #1 and Tiffany #2 as the most popular SM's female kpop artist by U.S. Yoona is also the most popular kpop artist in Asia (according baidu), Yoona has also the biggest fanbase for a solo sm female aritst in Russia. Yoona, Yuri, Hyoyeon are 3 of the most popular snsd members in Europe.