Just before i start i would like to give a shout out to ZiggyZ for inspiring these idea's i am about to portray and give him a + rep 
IF HE HAD A DEVIL FRUIT:
His devil fruit was the Yami Yami No Mi.
and here is why:
Has there ever been a Cliche moment in One Piece other than Protagonists somehow surviving or saving the day?
No.
So we can dispell all Mera Mera No Mi and Gomu Gomu No Mi theories.
Neither fruit has ever been mentioned to be Rogers power. Yet not a single mention of Roger came about.
Allow me to put things into perspective.
WBvsRoger happened A LOT. They where rivals to the bitter end , and i think Haki can only get you so far. Roger needed a very Overpowered Devil fruit power to compete with , tie with and defeat Whitebeard.
Blackbeards plan to become the pirate king was heavily involved with the Yami Yami No Mi , this may be because Roger previously had the same power he possesses now , the only difference is that Blackbeard may have experimented with it in ways Roger did not in order to become stronger by trying to take the Nullification of devil fruit powers even further.
He found out what it looks like , etc. Why would he need to do that? Because it was the Ultimate devil fruit power.The King of all devil fruits.
He could consume other devil fruit powers , nullify them etc. He seemed to know everything about the Yami Yami No Mi when he demonstrated its power to ace. Why? He experienced its power before.Name one deceased person other than Roger who has strength which is compatible with the Yami Yami No Mi , of course he wasn't fodder.
Not only this but he would make a great antagonist for Luffy , A man with the Pirate kings power is fighting Luffy for the title of Pirate king.
If this is true it is forshadowing that Blackbeard may be the King of the Pirates instead of Luffy , OR He becomes king of the Pirates and is then defeated by Luffy.
IF HE HAD A DEVIL FRUIT:
His devil fruit was the Yami Yami No Mi.
and here is why:
Has there ever been a Cliche moment in One Piece other than Protagonists somehow surviving or saving the day?
No.
So we can dispell all Mera Mera No Mi and Gomu Gomu No Mi theories.
Neither fruit has ever been mentioned to be Rogers power. Yet not a single mention of Roger came about.
Allow me to put things into perspective.
WBvsRoger happened A LOT. They where rivals to the bitter end , and i think Haki can only get you so far. Roger needed a very Overpowered Devil fruit power to compete with , tie with and defeat Whitebeard.
Blackbeards plan to become the pirate king was heavily involved with the Yami Yami No Mi , this may be because Roger previously had the same power he possesses now , the only difference is that Blackbeard may have experimented with it in ways Roger did not in order to become stronger by trying to take the Nullification of devil fruit powers even further.
He found out what it looks like , etc. Why would he need to do that? Because it was the Ultimate devil fruit power.The King of all devil fruits.
He could consume other devil fruit powers , nullify them etc. He seemed to know everything about the Yami Yami No Mi when he demonstrated its power to ace. Why? He experienced its power before.Name one deceased person other than Roger who has strength which is compatible with the Yami Yami No Mi , of course he wasn't fodder.
Not only this but he would make a great antagonist for Luffy , A man with the Pirate kings power is fighting Luffy for the title of Pirate king.
If this is true it is forshadowing that Blackbeard may be the King of the Pirates instead of Luffy , OR He becomes king of the Pirates and is then defeated by Luffy.