First, some considerations:

Please note that I am no developer, nor I have skills to implement or fix any of the problems listed here, my thoughts come from experience playing the game and past changes made by Nexon itself.

I do not have English as my mother tongue, so please don't mind some of the mistakes I may have committed down bellow in the post.

If you have more experience in any of the cases approached bellow, PLEASE, do share them in the comments bellow so we can have a better discussion here, I do not have a lot of experience with most of the classes I listed here, so i MIGHT, and probably will be wrong about some details.

These comes from heart and it's only intention is to promote a better gaming experience for everyone, so don't be rude/insensitive. I love Maplestory as a lot of you do, it's a bittersweet love, we all know.



Now, buckle up and bear with me:

As a game that is played in a global scale, we need a different treatment and some adjustments are required, most of the classes are created in KMS, as you may know, Korea is a small country, no matter where you at, you'll have a close connection to Nexon's servers and will have a small ping(9-20ms of latency).
The dev team in Korea happens to create characters with mechanics that relies on ping to work well,
while inside KMS this is not a problem, when this classes come to GMS most of these skills does not work well for the majority of people, this ping reliable mechanics stop working as intended and hurts the game experience of most of the player-base.

There is currently a number of classes in GMS that is currently not working in it's full potential unless you live in Nexon's server neighbourhood, I'll be naming a few here and will try to explain the why each of them is a problem and my thoughts on it's solution.

PATHFINDER
There is an issue with Pathfinder skills when using them back to back in a key or assigned to a macro.
Cardinal Deluge sometimes have a delay between you firing the skill and the arrows coming out of your bow.
This delay is not present 100% of the time, but its consistently there while levelling/bossing, when this delay happens and you use Cardinal Burst right after, it glitches and drops some inputs in your keyboard.

VIDEO PROOF:

As shown in the video, the input that is being droped is the jump input that I am performing every time I use the macro, in the video it makes the movement really clunk while attacking and double jumping because the game has to stop me from jump before the arrows from Cardinal Deluge is out.

When compared to using only Cardinal Burst standalone in a key, this input drop does not occur and I can move, jump and attack with zero problems.

How I think Cardinal Deluge works:
When using the skill, game will search for enemies in a certain range, then it fires arrows that seek enemies around you and hit it. If you use the skill with no enemies in range, the skill will do nothing but still consumes MP.

Possible cause of the problem:
I believe this is an issue caused by latency/ping, I play the game from Brazil, so my connection to the servers has a 200~ ping and floats around this number, this creates a delay when using Cardinal Deluge, and this "lag" created interferes in the macro, creating this input drop.

Possible solution:

Changing values in Cardinal Deluge so that it no longer waits for a server response, making it fire the maximum number of arrows it can fire(4) instantly after use(invisible), hitting enemies around if eligible, if no enemies are around the arrows can simply desapear in the air.

Changing values in Cardinal Deluge so that it no longer prevents the use of other skills until the arrows come out, making it no longer interfere in the use of other skills, creating a queue that will fire the seeking arrows in second plane once they are ready.

TELEPORT OF MAGES EXPLORER, LUMINOUS AND BATTLE MAGE.

There is an issue with some Teleport skills that affects the majority of the mages in the game, hurting the experience of people that want to play those specific classes from outside USA.

EXPLORER VIDEO PROOF:
LUMINOUS VIDEO PROOF:

In the video you can see a relatively high latency after using teleport skill and attacking, you can see that after I teleport I am unable to attack for a small period after the teleport ends, the character is able to move a considerable distance before the game allows him to start attacking again.

If you use the teleport skill consecutively you can see that there is a small delay between each use of the skill that is bigger than other classes such as Illium or Evan.

As cited above, this problem DOES NOT occurs when playing with mages such Illium and Evan, their teleport skills are codded in a way that it does not interfere in the usage of other skills, see the videos bellow:

ILLIUM TELEPORT:
EVAN TELEPORT:

How I think teleport works:
When you press the skill, it will teleport you in the direction you are walking and consumes MP.

Possible cause of the problem:
The problem is that the MP consumption is SERVER SIDED, so it needs to wait for a server response after every teleport you use, this creates the delay you can see in the videos of luminous and ice mage above.

Possible solution:
In the past, there was an issue that was present in the Flash Jump skills too, if you go back to a few years, there was some sort of fix on how flash jump skills worked, before it used to wait for a server response regarding MP consumption, just like mage's teleport.
The change made was that Flash Jump skills would instead use MP client-sided, so it no longer required a server response, allowing every class to flash jump around with no delay.
The solution here is would be to change all mages teleport skills to have it's MP consumption client-sided.
Making all teleport skills to stop interfering in the use of other skills, potentially eliminating the issue.

For battle-mages it is a different scenario because they use teleport as an active damaging skill, so I don't know how that change would be applied here.

MECHANIC HOMING BEACON
There is an latency issue with Mech's homing Beacon skill, it has a delay between you pressing the skill and it actually firing the missiles.
This creates a problem that holds you character back when you try to combo AP Salvo Plus and Homing Beacon together, as intended in the design of the class.

VIDEO PROOF:

As you can see in the video, when you try to use AP Salvo + Homing Beacon, it creates a huge gap between both skills, causing a lag that stops you from firing AP Salvo for longer than intended, this cripples Mech's DPM by a mile, because you loose so much time waiting for Homing Beacon to calculate its targets and fire the missiles before you can use AP Salvo.

How I think Homing Beacon works:
The skill will target monsters nearby you, once the game determines where the monster are, it will fire missiles that will seek the monsters and deal damage to it, if the target dies before the missile reaches it, the missile will rotate and try to find a new target for a short amount of time, disappearing if it does not find any target.

Possible cause of the problem:
When using Homing Beacon, the game needs to wait for a server response on where the mobs are at and how many mobs it will hit, because you have to wait for this response back from the servers, if you play in a 200ms~ ping scenario, you are stuck waiting for the server response before the game allows you to continue using skills.

Possible solutions:

Changes Homing Beacon codding so it will no longer wait for a server response to fire the missiles, it will now fire the maximum amount of missiles in the air(invisible), and after the server determines where each missile will land, it appears and hit the mob, this way you are no longer stuck waiting for a server response before you can use AP Salvo.

Changes Homing Beacon codding so it will no longer interfere in the process of using other skills such as AP Salvo, it will now be calculated in second plane and will fire as soon as the game determines each target, not interfering in the use of AP Salvo or any other atk skill such as 2nd or 3rd job.

BLAZE WIZARD AND ORBITAL FLAME LAG
This one is classic, the main skill for bossing and mobbing of this class, Orbital Flame, has a huuuuge delay in activation when played from outside USA(aka 200+ ping), you press the skill and it activates way later, you press the skill multiple times and it does not activate in some of the instances, it's a mess.

VIDEO PROOF:

As shown in the video, lag is the name and there is no need of further explanation.

How I think orbital Flame works:
You fire a ball of fire in front of you that bounces back to you after travelling a certain distance, hitting enemies both going and coming back to your character.

Possible cause of the problem:
I think the root of the problem here is how the skill is codded, if you fire the skill in a spot, and move from that spot, the skill will actively track your location and readjust the return location to where you character are at the moment, it will track you till a certain time and select that location as a return point to desapear, this creates a scenario where the game and the server have to be actively changing information, creating a overflow of information that gets stuck in a higher latency scenario, thus preventing you to use the skill again.

Possible solution:
This one is hard for me since it's a much more technical skill than the normal ones we have in other characters, it's much more complex.

Change the skill codding to be completely client-sided, changing the whole process of tracking your character location to be lag free, when you fire the orbital Flame, makes that it creates a fixed area of damage in front of you and any monster that is inside that area takes damage.

Change Orbital Flame to not require to go back to the exactly location of your character and only for the initial location it was used.

FINAL CONSIDERATIONS
This is a problem that we have been facing for ages in Maplestory, but Nexon has made some changes in the past that alleviated some of the problems and I believe that they are capable of much more.