At this point, I don't think DK needs too much to be competitive. IMO, he needs a bunch of cleaning up and QoL changes that would take all the random anti-jank out and smooth out his neutral a little bit. It's okay that DK can't really land and that he is extremely easy to juggle and abuse on the ledge. Weaknesses are just as important as strengths for the flavor and balance of a character. What is lame and not cool are things like the numerous moves DK has that are unsafe on hit at low %'s.
QoL Buffs
-Neutral B ledge grabs after punch (No more punch SD’s on block)
-Neutral B charge bug fixed (No more ghost wind-up, canceling wind-ups with shield stores charge properly)
-Giant Punch Super Armor in the air matches with Super Armor on the ground (Doesn’t lose SA when landing)
-Jab 1 inner hit always safe on hit/combos into jab 2, always jab locks (Changed to fixed base kb)
-FtiltU/S/D safe on hit at low %'s (Added base kb)
-Utilt safe on hit at low %'s (Added base kb, slightly lower kb growth)
-Usmash safe on hit at low %'s (Added base kb)
-Dsmash safe on hit at low %'s (Added base kb, slightly lower kb growth)
-Side B 1/3 sourspot safe on hit (Added base kb/angle change, knocks away Luma)
-Down B 1 safe on hit (Added base kb), does 1 pair of slaps per button press (Very annoying when it does extra and you get punished)
-Grounded Spinning Kong links more reliably (Fixed kb for all hits except final, not SDIable)
-Sourspot Punch kbg/dmg changed to normal for all Punches (No more awkward ghost hits)
-Ftilt whiff fixed (Shoulder Hitbox added or hitboxes moved and/or size increased)
-Dair/Fair are untechable against grounded opponents
-Air Down B changed to autolink angle so that it always links into second hit
With the above changes, DK would be a lot less prone to randomly getting punished for landing hits or silly hitbox issues. As a side bonus, he would also have some more appropriate reward on his laggy tilts/smashes and scale better with rage. People won't fly out of Spinning Kong's multi-hits or punish you for landing Side B/Down B. DK would be probably be somewhere around the top 15-20 with these QoL changes. Making DK a top 10 character around Mario's power level (probably the most reasonable power level to shoot for balance wise) would probably need
some of the big buffs below.
Big Buffs
-Ledge Roll goes farther and/or has slightly more invincibility frames
-Ledge Attack hits slightly earlier and also hits higher (on the downswing of his arm)
-Spinning Kong AC's like Kong Cyclone/Brawl's Up B
-Useless regular Dthrow is sped up and gets a hitbox that knocks Luma away/hits other people in teams
-Air Dodges changed to start and end one frame earlier (SH Air Dodge to Bair would work!)
-Spot Dodge starts frame 2 (Helps dodge lingering stuff better like ZSS tether grabs)
-Nair's hitbox size increased for the strong initial hit to cover his lower body better (Reliably hits PKT below him)
-Bair AC window starts earlier (Can AC with SHFF Bair)
-Fair has a normal AC window before it hits and after frame 42 (Getting hit while backwards and doing Fair won't be so silly)
-Uair AC's earlier and lingers a bit longer (Hits in front of him for longer)
-All aerials get slightly less landing lag
-Jab 2 sped up 2 frames to beat frame 6 shield grabs, hitbox lingers to beat dodges better like in Brawl
-Ftilt range matches the graphics of the swipe effect
-Dash Attack's strong hit goes through projectiles like Ike and Jigglypuff's Dash Attacks.
-Pivot grab given arm intangibility to beat spindash type moves instead of trading
-Dsmash starts frame 10
-Usmash scoops people into the main hitbox or just has size increased to hit lower/to the side a bit more
-Side B endlag massively reduced, buries for longer, aerial spike hitbox is bigger
-Air Down B landing lag and endlag reduced
-Cargo Hold time increased and Heavy Armor improved
-All item throw endlag significantly reduced (DK throws barrels with less lag than banana's and 99% of other items lol)
Obviously if he got everything mentioned above he might be like ZSS level silly, but individually they are all changes he could use.