The answer is Monet. I know it sounds outrageous and it is but there is some serious evidence backing this theory and I think it is very likely to happen. Let me break it down to you point by point.
1. Monet is alive: But didn't Monet die? No. Her heart was not fully stabbed. Caesar passed out before fully stabbing her heart as you can clearly see in the panel below:
But then why did Monet pass out? Well even Law passed out when Caesar pressed his heart so its possible that Monet just passed out in a similar way but is still alive as her heart wasn't fully stabbed. C-Block was also never shown to be fully destroyed. And anyway characters don't die that easily in One Piece so its very likely that she might be alive.
2. Monet isn't a bad person: We see in a flashback that she is being very nice to the children and doesn't seem like a bad person. I believe that she was the one who threw that "Don't do anything" note to Chopper. She was protecting Caesar because his SAD was important to Doflamingo. Other then that she didn't really care about Caesar and his giant project so she helped Chopper so he can cure the children.
3. Monet is an astronomer: Look at the book in the background of the panel below. It reads "ASTRO" which is short for Astronomy.
In another panel shown below there is this book in the background with "CLIP" written on it which is short for Eclipse. So Monet is studying books on astronomy and it all just points to the fact that she is an astronomer.
4. Strawhats need an astronomer: Ok so Monet is an astronomer, so what? For those of you who don't know, space has a big role in One Piece world. A complete article of itself can be written on this topic so I will just mention it in short here. One Piece world has six moons and it is a well known fact. Don't believe me? Look in the background of the panel below:
It is believed that many different species originate from space and there is a well established theory that claims that Celestial Dragons are from one of the moons. It was mentioned in the cover pages that the Skypian races are from the moon.
But these are topics for another day. Anyway, there is definitely going to be a space arc in One Piece and Strawhats are going to need an astronomer to go to space. It is possible that even Doflamingo appointed Monet because he needed her help for going to space.
5. Trends in One Piece: There are repeating trends everywhere in One Piece. It is full of foreshadowing and symbolism. Dressrosa arc is very similar to Alabasta in many ways. A schichibukai king trying to take control, the real king, reunion with brother etc. How things are going on in New World in general are very similar to how they happened in the Paradise. At the end of Alabasta arc Robin joined Strawhats and she was an ally of Crocodile. Monet is the Robin of Dressrosa. Characters of Robin and Monet are also very similar. So Monet joining Strawhats is going to be continuation of this trend of similar things happening in second half of grand line.
Also there is this pattern in the Strawhat crew that female members have more sophisticated jobs. Nami has the job of a navigator and Robin of an archaeologist. I'm not saying that men in the crew don't have important jobs but their jobs like cook, sniper etc. are much more simple as far as knowledge and complexity is concerned. So I believe the job of an astronomer would go to a female character. There are only two females right now in the crew so the next member should definitely be female anyway.
Now the only question remaining is how she will join the crew. Honestly, I'm not sure. There is a theory that she herself wished to go to space so she joined Doflamingo as she truly believed he will be Pirate King one day. But if Doflamingo dies in Dressrosa then she would have nowhere to go and will join Strawhats as they will seem to be the best bet to fulfil her dream. But this is a complete speculation and she might join in another way and some other event might occur which leads to her joining. But one thing that I'm sure of is that right now she is the best and most logical choice for joining the crew.
P.S. : I sourced a lot of this theory from Alpha2late17 so do check out his channel.
1. Monet is alive: But didn't Monet die? No. Her heart was not fully stabbed. Caesar passed out before fully stabbing her heart as you can clearly see in the panel below:
But then why did Monet pass out? Well even Law passed out when Caesar pressed his heart so its possible that Monet just passed out in a similar way but is still alive as her heart wasn't fully stabbed. C-Block was also never shown to be fully destroyed. And anyway characters don't die that easily in One Piece so its very likely that she might be alive.
2. Monet isn't a bad person: We see in a flashback that she is being very nice to the children and doesn't seem like a bad person. I believe that she was the one who threw that "Don't do anything" note to Chopper. She was protecting Caesar because his SAD was important to Doflamingo. Other then that she didn't really care about Caesar and his giant project so she helped Chopper so he can cure the children.
3. Monet is an astronomer: Look at the book in the background of the panel below. It reads "ASTRO" which is short for Astronomy.
In another panel shown below there is this book in the background with "CLIP" written on it which is short for Eclipse. So Monet is studying books on astronomy and it all just points to the fact that she is an astronomer.
4. Strawhats need an astronomer: Ok so Monet is an astronomer, so what? For those of you who don't know, space has a big role in One Piece world. A complete article of itself can be written on this topic so I will just mention it in short here. One Piece world has six moons and it is a well known fact. Don't believe me? Look in the background of the panel below:
It is believed that many different species originate from space and there is a well established theory that claims that Celestial Dragons are from one of the moons. It was mentioned in the cover pages that the Skypian races are from the moon.
But these are topics for another day. Anyway, there is definitely going to be a space arc in One Piece and Strawhats are going to need an astronomer to go to space. It is possible that even Doflamingo appointed Monet because he needed her help for going to space.
5. Trends in One Piece: There are repeating trends everywhere in One Piece. It is full of foreshadowing and symbolism. Dressrosa arc is very similar to Alabasta in many ways. A schichibukai king trying to take control, the real king, reunion with brother etc. How things are going on in New World in general are very similar to how they happened in the Paradise. At the end of Alabasta arc Robin joined Strawhats and she was an ally of Crocodile. Monet is the Robin of Dressrosa. Characters of Robin and Monet are also very similar. So Monet joining Strawhats is going to be continuation of this trend of similar things happening in second half of grand line.
Also there is this pattern in the Strawhat crew that female members have more sophisticated jobs. Nami has the job of a navigator and Robin of an archaeologist. I'm not saying that men in the crew don't have important jobs but their jobs like cook, sniper etc. are much more simple as far as knowledge and complexity is concerned. So I believe the job of an astronomer would go to a female character. There are only two females right now in the crew so the next member should definitely be female anyway.
Now the only question remaining is how she will join the crew. Honestly, I'm not sure. There is a theory that she herself wished to go to space so she joined Doflamingo as she truly believed he will be Pirate King one day. But if Doflamingo dies in Dressrosa then she would have nowhere to go and will join Strawhats as they will seem to be the best bet to fulfil her dream. But this is a complete speculation and she might join in another way and some other event might occur which leads to her joining. But one thing that I'm sure of is that right now she is the best and most logical choice for joining the crew.
P.S. : I sourced a lot of this theory from Alpha2late17 so do check out his channel.