What was Roger's power that is referred to as 'hearing the voice of all things'?
I don't believe the subject has been explicitly explained, but it is shown to be the ability to decipher any text/language.
.
Are there strong/influential characters that got turned into a doll by Sugar who are now human once again, are being remembered and might play a big role in the future?
We do not know all of the people who have been transformed, just the ones who are fighting Doflamingo's forces now. However, more may appear soon.
.
What was the timeskip like for Robin?
I don't believe we have seen Robin's experiences during the time-skip; although, information may arise.
.
What will Coby's final rank as a marine be, how strong will he be at that point and what changes will he (want to) make to the system should he become a(n) (vice-)admiral?
This is a controversial topic; many people believe Coby will see his dream through and become an admiral, whereas others believe he'll reject the promotion and remain a vice-admiral.
.
What happened to the remainder of the Whitebeard Pirates and when will they resurface? What have they been doing during the TS?
We don't know what exactly happened after Whitebeard's funeral, but it has been implied that they are still together. We do not know what they did during the time-skip.
.
What is admiral Ryokugyu like?
We have yet to see him; however, I believe Doflamingo remarked that he, like Fujitora, was very reckless with his power. (Correct me if I'm wrong)
.
Where is Aokiji and what's his role in the underworld and his business with Blackbeard? Will he ever end up as a protagonist?
The last-known sighting of Aokiji was Punk Hazard, I do not believe we have seen him anywhere else. It is implied by that Aokiji has joined the Blackbeard Pirates as when Jesus Burgess said he didn't trust Aokiji as he was a marine, Blackbeard said, "Well, you could say the same for Shiryuu." We don't know if he'll be a protagonist.
.
Who are the new Shichibukai?
Buggy, Law, and supposedly the unknown person who cut off the Z's arm are the new Shichibukai.
.
What is Blackbeard's crew like in terms of strength, commanders, devil fruits? (Apparently BB spent the TS stealing all sorts of powerful DF's and I'm dying to find out which ones he's already got his hands on) Also, has he found a third DF for himself or is he still searching for it? Which DF would he have/be interested in as his third (and final?) one?
We don't know the exact strengths and abilities of each crew member. We do not know of Blackbeard got another Devil Fruit. The third may only come as pure speculation.