Well...this idea may be completly random...but what about Luffy's second brother?
Sure, he is not in the story at the moment, but this is Oda and character was presented for sure and this is the right time for him to reappear (well..if he still alive)
So he will become the ghostrider.
I'm currently stuck between the following options:
1) Luffy - out of boredom/an accident, much like he got to eat the Gomu Gomu no Mi. But not without actually Oda explains why this wouldn't contradict Blueno's explanation.
2) Zoro - so he could have his flame sword anytime he wants, even by three. But Oda also could solve this by making Zoro learn Kinemon's art of sword...
3) Usopp - in some way to prove his manliness..
4) Fujitora - assuming he doesn't have any DF ability yet, I think a Marine admiral with a Logia DF wouldn't be wrong at all.
5) Whoever of the Strawhats gets it first, applying the rule of the Whitebeard Pirates ("Whoever non-ability user gets the fruit first has the right to get its power.").
6) It's going to fall into the hands of the Blackbeard Pirates.
So far Coby has yet to join the current arc at some point to make me consider that he'll eat it.
The last candidate: Luffy's straw hat!
So that when Luffy finally returns the hat to Shanks, Shanks has to let Luffy keeps it because wearing the hat will burn all his hair and Shanks doesn't want to be bald at all.
Well would love a touching moment where usopp gets his hands on it and luffy says go ahead, its ok if its you.
But it probably wont happen since it would destroy the character of usopp -_-
Would be cool to see nami wrapped in nothing but flames
My candidate is Kabuto. Before I thought Usopp, but I don't think a sniping ability would work well with an all-powerful logia. How silly is to snipe when you can incinerate someone on the spot?
The problem is that so far we saw objects "eating" zoans, which naturally gives them a mind, which in turn can be trained controlled by the owner. How would one control Kabuto's power if the weapon could not take orders?
I highly doubt Zoro or Sanji will eat the devil fruit, nor do I want to see them eat any devil fruit. Their being able to swim underwater helps set them apart from Luffy because they can get the job done underwater if need be (Sanji saving Luffy, Zoro slashing Hodi) while Luffy is restricted to air (bubble, sky, land, whatever).
I can see Luffy getting mera mera no mi, but he'll either keep it safe or keep it with him and put it in Ace's grave.
Though, I could see him eating it too for some reason, and not dying.
I believe it would be great for Luffy to eat Ace's fruit, especially since BB already has 2 broken abilities and he might also have more by now if his main DF alows such a thing. Luffy needs something to power him up.
Since Ace died, I always thought it would be a little funny if Luffy somehow got the Mera Mera no Mi and ate it becoming a burning rubber man.