I think that would be great as long she stays in Kara.
As you know that Taeyeon doing some solo, but she still part of SNSD. I admit that Taeyeon's voice is really beautiful even it beat Jessica's voice with slight difference. Taeyeon has so much power in her voice.
I think Gyuri could do the same too, but I still like her if she still stay in Kara, she could do the solo, but Kara is her family