Going back to your OP question... I've been out of Melee for too long to remember how all the matchups go for DK. I know the popular meme a while ago was that he did really well against space animals in general, but terrible against sword characters. I never played Brawl for anything more than party game status, so I have no concept for how that might have changed for that iteration.
But assuming DK still had issues with being zoned out by characters with disjointed hitboxes, and still had a lot of trouble getting punished and juggled, the devs have their work cut out for them in patching up DK for Smash 4. DK doesn't have to be good against the whole cast, but he shouldn't be mediocre at best against the whole cast, either.
I have a lot of hope in the new dashing roll attack. I've only seen it from the front, and I've yet to see it hit anyone, but if they did it right, it has the potential to allow DK a solid grounded approach, facing forward. That's pretty huge. I'm pretty sure all of DK's aerials have been shown at one point or another, though I can't recall seeing them all, but if they put some range and power and/or armor on Nair, I would love that. Bair approaches are fine, but it's so predictable and easily dealt with after a while. DK does well playing defensively against non-projectile characters to an extent, but going offensive and closing that gap is hard for DK. I never did any short hop Dairs with DK, but if his stomp worked kinda more like Ganon's stomp, that would be a fun punishing tool out of shield.
DK has long arms. I think giving him a tad more range on moves that aren't Giant Punch would be great. Ftilt and Dtilt, for instance, I could see being good for reaching out past DK's usual range, but they have issues. Ftilt doesn't really do anything to help set you up, and it has some lag. Dtilt comes out moderately fast, but it's such a pathetic poke that all it really does is psyche out your opponent. If the person you're playing has a strong mind, Dtilt seems too weak to do much. I seem to remember that DK has a lot of moves that are easily punishable due to ending lag. Fair is fine for edge guarding at times, for instance, but the lag on that thing is so stupid. The delay is also really counter-intuitive for the person using DK, which only serves to help the person playing against him.