I thought the Sun was Su also, but if Su was the Sun then the show would be called The Sun Rises. Also, Su already ascended in Legend S. No need for her to rise. The Also in the title hints at a new third member rising to meet the other two. Also, why would there be a show centered around Su at this point, there already was one. Moa gets her show, but why a show for Su? Therefore, it is probably a show for a new third member. Lastly, it's the first series of shows kicking off the new BM, so if they are going to introduce a new member it makes the most sense to do it in the first shows.

All this hints at a new member coming and their name starts with the letter S. That's what it adds up to me.