In the empty spot will be Moonvine and Tachibana (the next Event Girl).
I give new players that advice all the time, try to find 5 girls of each type and settle with those. It will help you to spread the Manyuu's around the same. When replacing you can normally just go with rarity but when you are inside the same rarity, maybe got a new 4* out of the Normal Gacha, try to look at the girls in detail. You have the information on the wiki, might as well use it, and look what Battle Skill do you like more, maybe the character shares it with another character of lower rarity, or see if the abilities sound useful to you. Do you want a tanky, balanced or glass cannon like character?

Sure you can just press "Sort after Total Power" and live with the fact that maybe you now have 7 Magic and 3 Pierce characters, but the wiki should maybe not be used just for the additional forum. :P