As One Piece's "Wano" Arc is nearing an end, it seems like the Ninja-Pirate-Mink-Samurai Alliance is set to take a decisive victory over the Beast Pirates and Big Mom. If this happens, then it means that there will be few left in the world powerful enough to give the Straw Hat Pirates a hard time. If Luffy really can beat a nigh-invincible Emperor on his own, then the only ones left who can stand up to him are the World Government and other Emperors.
It seems likely then that the next big obstacle for the Straw Hat Pirates will be Blackbeard. If Kaidō and Big Mom fall, there are all sorts of ways for him to capitalize on it. He would also have something to gain from getting the jump on Luffy and his friends. If Blackbeard plays his cards right, which he usually does, he could become a threat on par with the World Government.
The first thing to consider is what happens if Kaidō and Big Mom lose. It's possible they'll be arrested and taken into custody by the approaching Marines. Even if they aren't taken in, they should be severely weakened after the intense battle on Onigashima. Blackbeard can use this opportunity to take out two Emperors at once and take over their territories.
There's also the possibility that Blackbeard will want Kaidō's Devil Fruit for himself. It's not yet clear how or why he can take in multiple Devil Fruits, or even transfer powers into others, but it seems natural that he would want to add the ultimate Zoan Devil Fruit to his collection. If he gets this Fruit, then he will have the strongest Devil Fruits of all three types under his control; Zoan (Fish-Fish Fruit, Model Azure Dragon), Paramecia (Quake-Quake Fruit), and Logia (Dark-Dark Fruit).
The Road Poneglyphs are another potential prize for Blackbeard. If one can gather all four Road Poneglyphs and decipher their combined message, they can find the way to Laugh Tale and the One Piece that Gold Roger left there. As it stands, three of the four scripts are about to be in Wano all at once; Kaidō has one stashed away somewhere on Onigashima; the Straw Hats have the rubbings from Big Mom's Road Poneglyph; Zunesha, and thus Zou's Road Poneglyph, is currently en route to Wano. Kaidō himself even says that whoever wins at Onigashima will be that much closer to acquiring the One Piece. If Blackbeard were to attack, he wouldn't have a much better opportunity to gather these ancient writings than right after the raid on Onigashima.
Furthermore, should Kaidō and Big Mom fall on Onigashima, which one of them already has, then it would be important for the remaining Emperor to take out the young upstarts responsible for this. If the pirates of the Worst Generation are really going after the Four Emperors, then they'll likely challenge either him or Red-Haired Shanks next. It would be in Blackbeard's best interest to nip this problem in the bud.
Blackbeard also has something to gain from attacking the Straw Hat Pirates in particular. In addition to their copy of Big Mom's Road Poneglyph, there's also the potential of acquiring Luffy's Gum-Gum Fruit. He's already shown interest in it back in Dressrosa. Furthermore, it's recently been revealed that the World Government is concerned with the hidden abilities of this Fruit. Since Blackbeard seems to know more about Devil Fruits than the average person, he may already realize the terrifying true potential of the Gum-Gum fruit and thus want it for himself.
It's not unreasonable to assume that Blackbear will be the next on the list of enemies the Straw Hats will have to take down on their journey to find the One Piece. He's already a relevant threat as an Emperor of the Sea. With the coming change in the balance of power, he has many different means of becoming an even bigger menace. Unless the Marines have something to say about it, he could even show up right as the "Wano" Arc is closing up. If the Straw hats aren't careful, Blackbeard can turn their adventures into a nightmarish, unmitigated disaster.
