Makarov isn't coming back, the next chapter is titled "The Seventh Guild Master". The only person I can see is Laxus as the master, I hope it isn't Lucy. I don't mind her, but Laxus has been foreshadowed as the next master. Lucy would come from nowhere.
Yeah but Makarov also happens to be the third and sixth Guild Master. I hope it's not Laxus, while it has been foreshadowed that he will be, as he seems to be more privvy to the secrets of the guild, he's not the best leader, a leader needs to show good teamworks and understanding, and Laxus hasn't been shown to be best people person.
I'd rather see Mira, as she does possess the skills to delegate, she is powerful and she doesn't go on as many missions as the others so would not be as hindered as the others by not having the time to go out on missions being stuck with paperwork.
I wouldn't even mind Lucy, she's got the brains to run the guild, and I still can't shake off the feeling she's related to Mavis somehow. though that is just a gut feeling, nothing in the manga seems to suggest such a thing. I don't think the most powerful person needs to be the leader, while it shows strength of the guild, it does't always make sense to have your strongest piece stuck being a desk doing paperwork.
I digress though it will likely be Laxus, he was the only one that was hinted towards in the manga, the strongest being when they were on the island and it cut to him when they were in trouble.