I want to make a thought experiment. Say a powerful character like Roger used one of the known DF powers:
Gomu Gomu: Stretching gives range, but it's true power is elastic potential. He can hit very hard with pure contact force. Haki would allow even more potential including nullifying DF protection and negating physical hits, and increasing overall damage.
Bara Bara: Can't be cut. No exceptions? Mihawk didn't hurt Buggy, but Mihawk didn't use a special technique. Can attack over long distances. Can fly to certain extents. Can use weight reduction to increase running speed and jumping height. Floats silently. Haki usage unknown.
Fuwa Fuwa: Jumping directly to Shiki's ability, like Shiki, he makes things float, and practically anything. Considering Shiki lifts and throws battleships like small rocks, it's terrifying.
Gura Gura: If Roger had Whitebeard's power, he'd be just as powerful, maybe more so because he wasn't as old when he died. Can shake things, sometimes to the point of shattering, even the air.
Moku Moku: Smoker's fruit. To be honest, Smoker's doing quite well with it. Unless he goes against Haki. If Roger had it, he would counter the Haki with his own. Produces, and spread smoke all over, and is able to grab and capture things in smoke. Turns into smoke to defend.
Hina's cage fruit: Captain Hina can bind people's limbs on contact, and can even produce cages. She was too slow to capture Luffy. Roger has speed, logically since Rayleigh, Shanks, and Shanks' top crew members can move faster than human perception. The cage's toughness depends on the person's mental strength.
Hito Hito: According to a SBS explanation, when a human eats this fruit it increases his abilities as a human. Including Haki?
Bomu Bomu: Explosions. 'Nuff said.
Kilo Kilo: Weight manipulation on self. With the right skill level, Roger can kick as hard as Kizaru, and move extremely fast. He can easily change direction of motion and the duration of movement, so he can hop all over the place while breaking everything in one hit.
Bane Bane: Bellamy sucks, so he never used this to its potential. It's almost just like the Gomu Gomu fruit, but springs are made of metal and stores energy in pressing and stretching. I think Oda chose Gomu over Bane because he came up with Gear 2nd and 3rd, which can only work on rubber.
Regular Zoans: Zoans increase physical strength, agility, size, and senses depending on the animal. With Haki, Roger would be like a Rokushiki user, but not limited to 6 forms.
Suna Suna: Crocodile can turn into sand at will and as a reflex, and can produce and control sand. In addition he can dry things with his right hand. Only his right hand? The admirals can produce their element from any part of their bodies. Crocodile is an amateur by comparison.
Goro Goro: Easily one of the most powerful and terrifying DFs. Reputed as the strongest. Only weakness is rubber. Actually scratch that. Enel could have made a lightning sword than slices and burns, if only he could focus his electricity. Even with Mantra, he had very little offensive ability against Luffy. He had that pole that he molds into a trident, but Luffy's not afraid of that. Other things that can be done: turn into a magnet, cause static electrical attraction and repulsion, create electric fields that fry everything too close to him, increase muscle strength (Enel did), increase reaction speed (done), blow shit up (done), enhance detection (done), increase computation ability (done), shoot ion beam lasers (done), self-resuscitation (done), light as cigarette (done lol), list goes on.
I'll stop now because I'm tired.