Combo's are fine... I guess?
Like, they're not awful. But they're not particularly noteworthy, either in terms of fun or engagement.
Since, pretty much they're just 123 with increasing damage for each subsequent strike. The exception is Monk whom merely gets access to skills which have different effects as opposed to just getting increasing damage.
Meaning that when you get down to it, it's kind of like you're just spamming a single button for generic filler only you have to use 3 buttons instead of 1 and you also can't use certain skills mid-chain because they interrupt combos (Such as using a ranged attack, a PLD using Shield Bash, a DRG using Heavy Thrust, using an AoE skill etc)
As far as improvements go?
Well, reducing just how much potency is gained per step and instead focusing on accessing skills with alternate effects would go a long way. So that all 3 strikes in a combo feel good to use and not weighted towards the latter skills. An example is instead of WAR's Heavy Swing > Maim > Storm's Path being 160 > 200 > 280 it's more like 190 > 210 > 240. Where much of the benefit from the combo is from alternate effects, such as how Maim and SP give Gauge.
Outside of that... I don't know... Personally, my favourite combo system was the one from City of Heroes and the Dual Blades powerset. Which took the form of:
9 skills, 4 combos:
Nimble Slash - Low damage, very fast cooldown
Power Slice - Moderate damage, fast cooldown
Ablating Strike - Moderate damage, enemy defence down, fast cooldown
Typhoon's Edge - PBAoE moderate damage, slow cooldown
Blinding Feint - Moderate damage, self damage and accuracy up, slow cooldown
Vengeful Slice - Moderate damage, enemy knockdown, moderate cooldown
Sweeping Strike - Cone superior damage, moderate cooldown
One Thousand Cuts - Cone superior damage, slow cooldown
Combos where 3 skill combinations:
Weaken - Enemy accuracy down and defence down:
Nimble Slash > Ablating Strike > Typhoon's Edge
Empower - Self damage and accuracy up:
Nimble Slash > Ablating Strike > Blinding Feint
Attack Vitals - Cone DoT
Ablating Strike > Vengeful Slice > Sweeping Strike
Sweep - PBAoE knockdown
One Thousand Cuts > Power Slice > Typhoon's Edge
If I recall correctly, the combo effects also dealt damage to targets they hit too (Except for Empower obviously)
But the general gameplay was that every skill had a different CD, meaning what combo's you did depended on what skills were available as well as what effect you wanted. With downtime being you just using the lower CD skills as filler until you get a combo available again.
In the end, you had a nice flow to your combat, where you were constantly mixing up your attacks based on what was happening and what was coming back up (Especially when you'd got some CDR mods put into the longer CD skills so they would come back up a lot faster) as well as getting some nice feedback from actually landing the combo, via some impactful effects rather than "Just X more damage"
Whether or not it's feasible to take some elements of this design and incorporate it into XIV I don't know...
Possibly?
Like, if we take our standard 3 skill combo we can have the following options:
123
321
213
231
312
132
That could give us 6 different "Combos". With a system much like how Mudra work. (There's also more if you allow for repeat numbers... Like 121, 131, 112, 113 etc)
If you wanted to get crazy, you could even have each skill change depending on what combo's you're doing.
So for example on a Warrior: If you start off a combo with say... Maim, your Heavy Strike could become say Skull Sunder and Storm's Path become say, Fracture. Then after using the next skill in the sequence (Skull Sunder/Fracture) the remaining skill then becomes something else, like maybe after Skull Sunder, Storm's Path becomes Butcher's Block while after Fracture, Heavy Strike becomes Crushing Strike.
Which would allow for a plethora of skills, within the 3 button combo that we're used to. The only change required to set it up, would be to change the current "Combo" skills into having a base potency equal to the current combo starters (I.e. Maim/SP being 160 potency same as Heavy Strike)
With the ideal outcome being less focus on just 123 ad infinitum and more shifting what combo you do based on what you need at the time. Such as DoT application, buff upkeep, debuff application, healing, mitigation, conditional bonuses (I.e. "Deals more damage to targets above 80% life/below 20% life") etc.