With more than 50 Copy Abilities out there (not even including the mix ones), there are bound to be similarities. Some are obvious, others are more subtle, but regardless, they're there. Some could be replaced, others could be merged, and so forth. I think this would be a great topic of discussion.
The similarity that I feel is most obvious (outside of "clone" Copy Abilities e.g. the Sword and Bomb variants from Squeak Squad) is Throw and Suplex. Both heavily rely on throwing and have blue ribbons to go with it. It'd make sense to combine the two, and it wouldn't be hard either. Throw could be initiated through pressing the attack button while idle, in the air, or during a walk; it'd work as an inhale like usual. Suplex would be activated via dash attack, as Suplex grabs opponents by lunging. All attacks between the Copy Abilities would be intact, except for one of the lunge attacks, as Suplex has a standard and a fast one. However, that won't be a big deal, and the inclusion of the pinpoint kick really helps. The only problem is how would they work in modern Kirby games. In modern Kirby games, the aim system is not very precise instead of limited to forward, up, and down; however, the grab system is slightly different. You're still invincible, but now other enemies or bosses can prevent a throw by bumping into you; you'll deal damage, but you won't get to throw, and the damage will be less than desirable. Throwing usually also hits once, whereas Throw and Suplex can hit multiple times. One of Throw's ways of dealing damage is just holding an enemy and letting bosses touch them, dealing multiple hits; I don't want this mechanic to go to waste, as well as Throw's ability to hit multiple times from the throw itself, something that is now unfortunately absent in Hi-Jump. Beetle could also replace Suplex/Throw, but I'd rather that not happen, as I really do love Suplex and seeing it leave would be very sad.
The next ones are Cupid and Archer. Both use bows and arrows as their main utility. Cupid is a more narrow Copy Ability, focusing on a particular archer, whereas Archer is more broad, focusing on archery in general; however that doesn't necessarily make one more or less likely to return. If Cupid were put in a modern Kirby game with the new aiming mechanics, better damage and charging, it could probably rival Archer very well. Its arrows could also receive homing abilities, similar to the Angel Bow from Kid Icarus: Uprising; I'd also like the idea of fire/bomb arrows. Its wings are also nice. They could be used like Wing Kirby for fast flight, or maintain their standard usage of being able to do precision hovering, the latter of which sounds better. While Cupid wouldn't have Archer's stealth moves, it could still incorporate some of its moves into the move-set. Archer wouldn't be able to do the same regarding Cupid's moves. Really depends on what you want: hovering and more arrows, or stealth invincibility; triple shot, or defensive strike.
A less obvious one is whip and Yo-Yo. Both use rope contraptions to deal damage, and use directional attacks. They also have grabs and can perform a special attack using down-up-attack, both visually similar to each other. Yo-Yo would be slower but more powerful, but doesn't have a combo like Whip. It's really up to preference at this point: Indiana Jones, or EarthBound. Should Yo-Yo replace Whip, the ability to grab objects behind walls should be kept intact.
Now there's Jet and Hi-Jump. Both abilities have a super jump attack, Hi-Jump being fully based on this, with Jet only having one such attack. Hi-Jump could be fully incorporated into Jet's move-set though. Jet's super jump could instead be initiated by an up attack instead of a double jump at full charge. It could also be charged up like Hi-Jump to give it more power, and a downward variant could be added as well. Jet could probably overtake Hi-Jump entirely through that, as well as having quick horizontal movement, a Jet cracker, an a very powerful grab attack. It could work, but it'd be a shame to see Hi-Jump gone, being one of the first abilities ever and having a super hero cape and Hermes' winged circlet.
Now for some more interesting ones.
Double-Bladed Laser Sword and Thunder Sword. Thunder Sword has a cool name and adds even more greenness to Kirby's already green cap. However, most people upon discovering Kirby wielding a Lightsaber probably loved it. It'd be nice to see Lightsaber (going to refer to it as such because it's easier) return in a Kirby game, but it'd be tricky to design a move-set for without it cloning Sword. However, it could be used to reflect projectiles, as well as utilize some lightsaber attacks used in Lego Star Wars; unfortunately, saber clashing wouldn't be a thing though. It could also be swapped between a standard one and a dual-sided one by a down-attack on the ground; this is somewhat similar to character transformations in Melee and Brawl.
Fire Sword and Sword of Fire. Sword of Fire is interesting as the blade is triangle shaped instead of the kite shape. It also has an active hit-box even while idle, however this is rather outdated outside of grab moves. A swift slice could still work, as well as holding it above the head. The throw would be interesting as well, but whether or not it'd forfeit the ability like Hammer is a different story. It will still need for moves though, and preferably not clone Sword.
Bomb and Crash. Not a merge I want, but it's possible. Bomb could have a forfeit technique that would work like Crash. It could also be activated by the self-destruct attack. Seriously, nobody uses that attack, and it dealing self-damage in Super Star (Ultra) and Squeak Squad is a bigger incentive not to use it. However, if holding the bomb for periods of time would make it more powerful vs spamming it, holding your bomb could be a good thing. The self-destruct technique could be potentially useful by this mechanic, should it happen.
Now for some incorporations and modifications.
Spark field: In Squeak Squad, Spark would charge up static to make a bigger spark field; the scroll further upgraded this, along with making it more like Plasma. It's a bit of a shame this didn't return, but it could still work. Spark attacks could be activated by being idle, walking, or in midair, similarly to how Throw would be activated as mentioned earlier. Plasma attacks would be activated while dashing.
Stone boulder: Stone is a great ability, and one thing I like about it is that it goes down hills; however, I'd like to see the roll from KDL3 and KatAM return instead of sliding down more slowly. This could be incorporated with Turbo Stone, turning into a rounded rock instead of the flatter ones; this could still have variants though, probably all unique to Turbo Stone. Walking as a stone statue would be nice, but in today's games it'd probably be pretty broken.
Beam and Parasol upgrades: The more powerful Beam whip has only appeared in Squeak Squad, as well as Parasol's mini stars it shoots. It'd be nice to see them return, but they would be a little awkward; incorporating the Beam one may be difficult.
Water bubbles: Bubble Kirby has only appeared in Squeak Squad, for a rather obvious reason. However, it would be nice to incorporate its standard bubble attacks with Water Kirby.
Clean: I just want this ability to come back. It hasn't found much use and was going to appear in Kirby GCN, but of course that got cancelled. It's a real shame, and I feel the ability has great potential. However, he'll have to stick with the broom; no plungers or buckets.
UFO: Also a highly wanted ability. I'm sure various people got a bit bummed out when they found out Dubior only gave Spark, myself included. Hopefully it'll get the Squeak Squad treatment and be a normal ability instead of one that disappears after a level completion; don't worry, they'll be able to fix the ladder problem. If Kirby can be puffed up from a mint leaf and go through platforms at the same time, surely they can do the same with UFO.
Fire KDL2/3: I know it's called Burning, but that's beside the point. It'd be cool to see Fire Kirby able to perform a dive-bomb attack as performed with Coo and similar to Wing. It'd also be nice for Fire to shoot ranged projectiles, as used by Kine and Nago; however, being able to incorporate one or both could be difficult, as Fire has an aiming system, but I suppose holding the directional input before pressing the attack button instead of vice versa could work. Fire walls could be by downward input and fire balls by forward input as a "walk" attack. ChuChu's is cool, but I don't find it possible to do as Kirby doesn't have a hot-air balloon with him.
Spark KDL2/3: Spark can still shoot lightning (KRtDL), light up rooms, attack while on the run, and shoot lasers. The Beam attack could be incorporated by a quick tap instead of holding the button, similar to UFO. Note that I don't have any intentions for Spark to replace Beam, or Laser for that matter.
Ice KDL2/3: Ice can still surround itself in a blizzard, but it's lacking other techniques. Ice has rarely been able to aim itself, and I feel that should be added back so if may further compete with Fire. Being able to freeze while on the move would be useful, but it may be broken (talking to you Kine from Dream Land 2); then again, Spark has it, so maybe. Being able to propel itself into the air would be a cool addition (puns not intended), and would probably work like the Water Fountain attack, but with Ice. Snowball throwing would also be a cool addition, but I don't know exactly how it'd work. Perhaps a downward attack input while grounded.
Fire K64: It'd be nice to see the fire on his head actually damage enemies instead of being just a hat. Charging up the fireball attack would be great as well.
Needle K64: It may be awkward, but shooting a drill from his hand may work.
Bomb K64: I have no idea how, but I think it'd be interesting to have him incorporate different bombs into his move-set. Fireworks could have more range but less power, dynamite would be more powerful but could inflict self damage (don't know if that party hat will help protect him from rocks though), and missiles could home in on enemies, but have less power. Exploding Ninja Stars will be left for Ninja
This is... a lot to take in.... I didn't entirely think this through.... If you have any idea regarding possible Copy Ability merges or adding attacks to a move-set, feel free to include them. Here's your list of Copy Abilities for reference.