To OP, I personally would give the nod to Elema. She is one of the top TC20 tanks and can hold her own as center until the higher cups. You have joon and khionna, two future attackers that will compliment elena. Though elena is a solid center with high tiles for blue titans, you give up versatility on your offense teams with her.
Zim is more versatile, especially on offense, so I wouldnt blame you for going with her. Less of a center, but more well rounded.
Mitsu is more unknown, and seems geared towards the center position. But her blue protection really makes her niche. Given this is your first 5*, I’d go with a hero that you know what you’re getting.
And to echo another response, you don’t have to make a decision today. Pick one and level it to 3-70. By then you should have a better feel on whether it makes sense for your play style and other heroes whether or not to continue to 4-80.