@SuburbanErrorist:
Oda has certainly not done a good job in displaying Law as Luffy's equal in all the fights and everything they've been through together. Law could barely do anything to Doflamingo and believed Luffy would be the one to take him down.
You do realise Choppers goal is to cure any disease there is, right? It doesn't clash with Chopper.
Using chopper to say he fixes bad injuries so he's a surgeon and Law can't join the crew because of it isn't a good point since Chopper wasn't there when Zoro had to stitch up Mihawks wound.
As I said Laws purpose of joining the crew would be on the grounds of understanding the Will of D. The very reason why Corazon wanted to protect him from Doflamingo initially.
Only time will tell.
–- Update From New Post Merge ---
How many people thought Jinbei would join the crew originally? Who thought there would be a need for a Helmsman? How many people actually theorised that? Laws role if he were to join the crew would probably be the tactician.
Luffy has been consistently getting stronger since he has entered in the new world, whereas Law has remained stagnant. Jinbei originally was stronger than Luffy, now Luffy has surpassed him. I don't think anyone can regard Law as Luffys equal.
Law and Luffy are equals because they are in an alliance as equals.
It has nothing to do with strength. Luffy treats Momonosuke as an equal since he's technically the head of the Samurai in the current alliance. Chapter 819 was all about Luffy telling Momo to step up to his level.
And Law is not needed as a tactician either. Sanji, Jinbe, Usopp, etc. are all capable of formulating plans. His potential role in the SH is redundant.
And Law has never shown any intention of leaving his crew as well, unlike Jinbe. As shown in the most recent chapters, he will prioritize them over the current alliance. Again, this is not like the Fishman Pirates. The moment Fisher Tiger died, the crew more or less crumbled. Jinbe was the Jean Bart of the Fishman Pirates in this case. He couldn't keep the crew intact, which was a major issue.
Plus, Bepo considers himself above Jean Bart in the crew hierarchy. It's unlikely he would accept anyone but Law as Captain, since they're childhood friends. And considering all the steps Law took to protect them from Doflamingo, it's unlikely he would leave them behind.
–- Update From New Post Merge ---
The last crewmember is probably yet to be revealed. If Oda had any intention of having Law join, he likely would've given him a different profession to begin with, in order to make him as distinct as possible.
For now, he's content with just letting Law tag along as the guy that reacts angrily to the SH's antics.