He currently is holding hostage Caesar Clown. Caesar has enough expertise about devil fruit to implant them in inanimate objects, and extract them when desired. He also is the only person in the world capable of creating SAD, the key ingredient in making artificial devil fruit. The Strawhats are looking for the factory where those artificial devil fruit are produced. There is someone there to either extract the gomu gomu no mi from Luffy, or "safely" implant a second devil fruit power, and they are headed to a facility potentially containing supplies and equipment Caesar would need to do this. The plot has converged for Luffy to either swap powers, or gain a second, without the normal risk.
Eh, it seems like you are over estimating Caesar, I thought it was clear from Chopper's reaction that Caesar's work on SAD and SMILE was unimpressive since he was only working off of Vegapunk's notes. I really doubt Caesar has the power to extract DF powers from people or give them a second (real) fruit, otherwise, why wouldn't Dofla just take the mera-mera for himself? If someone has that power, its Blackbeard. Jimbei said that Blackbeard's crew was going around stealing fruit powers, if Caesar had any connection to that, then he would have brought up Blackbeard's name when he was trying to intimidate Luffy.
Mera mera no mi is one of the stronger known devil fruit, comparable to Kuzan's hie hie no mi, if used to it's fullest potential. I can't imagine Luffy's third in command gaining a fruit that powerful. Sanji then would be arguably the strongest Strawhat, by a fairly wide margin, with the mera mera no mi, armament haki, observation haki, powering Sanji's "black leg" to almost obscene levels. The same could also be said about Zoro. Add mera mera no mi to his swordsmanship and haki, and he arguably is the strongest Strawhat, if not the strongest over all character.
If Sanji gets the mera mera no mi then his kicks would be reinforced with armament haki, wreathed in pillars of flame. He also would be a logia user, only other haki users would be able to defeat him. Giving Sanji (or Zoro) so much raw power would be an ass pull of epic proportions. The combination of his martial arts ability, haki, and fire logia would give him the potential to be an admiral level character. It is early enough in the story that Sanji would have the time to gain experience, hone his ability, and meet that potential. The only thing capable of stopping him would be plot convenience. I have to doubt Oda would take that route.
You seem to be thinking about the first half of the Grandline. A lot of emphasis has been placed on the vulnerability of Logias in the New World. Pecoms was the first to criticize them, after one hit KO'ing Caribou, and then Vergo went more in depth during his fight with Smoker, where using his powers actually just made him a bigger target.
Although I don't really want to see anyone get the mera-mera (I kind of wish Oda had not reintroduced it to the story), I don't think giving Sanji (or Zoro) the mera-mera would make them unstoppable, or drastically alter the power balance of the monster trio. Intangibility isn't an unreasonable boost for any member of the monster trio since fodder aren't supposed to stand a chance against them anyways, but strong opponents will still be able to injure them now that everyone has Haki. I actually kind of agree with another poster that Sanji could use it to keep up with Zoro and Luffy, who seem to be widening the gap between their strength and Sanji's.
That being said, there are two very good reasons that Sanji won't get the mera-mera:
1) He already stated that the only fruit he wants is Absalom's.
2) Nami doesn't have Haki, if Sanji becomes a logia, Nami won't be able to hit him anymore and the plot won't allow developments that ruin fun things that Oda likes drawing.
