Koala is going to be the next Strawhat and you can take that to a bank with a Dooon!!!
I dont like writing essays but still let me give you reasons why- (I always got R in my all English Essays becasue of my spelling so please don't scratch the wound by correcting me and Thanks)
===She was a beautiful and cute at 11 so she will be hot and sexy like nami and robin by now at 23 (this is the most important reason)===
===Strawhats has 2 female crewmates and 7 male (ya chopper is the maan) so they need more female in the crew (you owe me one Sanji)===
===Koala had a horrible and painfull past just like rest of the strawhats and she might also have similar taste in dream like "all races treat eachother equal" ===
===She lived with sunny pirates for long time so she sure will be strong and courageous from inside like rest of the strawhats (i dont know brook's past but i guess you will also be courageous)===
===I am sure this right now she is some where on the fisher Island and she will show up any movement with some crazy fighting style or crazy devil fruit power (i want her to be a Swordwomen or with Water-Water Nomi No fruit)===
==='Just like Shanks means alot to Luffy then 'Jaguar D. Saul to robin and Franky had Tom-san, Koala has Fisher Tiger to look up to and admire===
well there are more resons then these and i know that you guys with find more if you think but the overall reason is that more strawhats means more action and more fun.