Reasons why the current system is kind of meh:
It's very spammy, basically you just spam KITO/Flex and go for a big cleave or even just KITO chip damage, or you spam things like quick barrage/full circle/sting/etc and go for chip damage with sword armor penetration. There isn't really any unique skills, almost all lack depth aside from Opportunity Knocks and maybe Dash which isn't good enough to actually use (IMO). Most other skills simply lower a defense, or raise openings.
The deck system allows for only the most optimal skills which doesn't allow for cool/unique situational opportunities. For example, sometimes an animal will be hitting your opponent raising them high on a color that you can't attack because you're on the wrong deck. Moves that could sometimes be cool such as Steal Thunder can't be used because the opportunities you would use it in are too rare to justify a deck slot. This contributes to a very narrow/repetitive system.
The four color system doesn't really feel enjoyable compared to a single attack/defense bar system (and is more confusing to noobs). It just allows for more repetitive spam of the same attacks over and over without much thought and the attacks will just build up small amounts of damage until it eventually KO's you regardless of how well you're defending. This also limits the possibility of cool skills that legacy used to have such as evil eye, battle cry, etc. A single attack/defense bar system allows for less chip damage spam as your attack weight will be removed/reduced after each attack.
The IP system isn't integrated well. Making it so optimal defenses like ZigZag/Artful Evasion give your opponent tons of IP kind of removes the purpose of IP even mattering. It also contributes to the group with more numbers maintaining a consistent advantage. For example, if I'm getting chased by two people and one of them is always hitting me I'm never really getting an opportunity to gain IP on them and they're gaining a ton on me. If I can't gain IP on them I have no opportunity for counter play and am at a permanent disadvantage essentially. A group with more numbers should of course have an advantage, however, the advantage shouldn't be so significant that counter play is basically impossible.
Currently the only counter play for fighting greater numbers than yourself is having significantly higher stats than someone, or using the combat meditation maneuver to opportunity knock them at an ideal time. If a player knows what they're doing or isn't careless this is essentially impossible and it's already extremely difficult to even get enough IP to pull it off.
Ways PVP could be changed/improved:
Firstly reducing the impact of outnumbering someone in a fight. Ideally there should be plenty of opportunity for skill to outshine numbers/stats. While the efforts towards stat equalization have been nice, it'd be great to see efforts towards number equalization as well. One thing that could be done for this is making it so the attack weight/damage of attackers is reduced based on how many people are aggrod to a target. The idea behind this would be so that if you're in a 6v10 for example the 10 people don't have 100% attack weight/damage, they have a reduced amount to better match the amount of people they're fighting. The weight/damage should of course still be higher than the 6 in this example, however, it shouldn't be so high that there's nothing the 6 can really do. There are of course better ways to implement it other than the amount of people aggrod to a target, perhaps it could perform the calculation upon receiving an attack from an opponent with a different GOBID, etc. The specifics can of course be debated/discussed, however, the overall idea would be a huge point of balance in my opinion. Albion Online does something similar very well IMO. Perhaps there needs to be some way to force people you're fighting with to be in your party and implement it on a party based system.
The deck system should either be removed or significantly increased. Having the inability to act on situational opportunities completely removes any surprise/thrill/variation outside of the meta. It also leaves 90% of the skills being completely unused (think would be cool but you need take aim to use it and you have limited space). I believe the way you guys have implemented the deck system on customizable keybinds easily allows for more combat skills to be in the mix at once too without much issue.
There should be way more skills (or old useless skills should be removed in favor of new/better skills) with significantly more depth. Legacy had some incredible skills that could completely change the tide of a combat situation out of nowhere. To name a few, No Pain No Gain which would raise your attack and lower your opponents defense with the trade off being you lose 50% of your SHP, this is basically a full yolo move that allows for a pretty extreme outplay on an opponent with a high degree of risk. Battle cry which would raise your attack bar back up to 100 but lower your defense by 50 and operated on a very small cooldown, basically another high skill/timing outplay skill with a high degree of risk. Evil eye, reduce an opponents entire attack bar, resulted in potential life-saving last second uses where you could be certain you'd finish someone and they surprise you or you just barely manage to live. Throw sand, reduce the opponents IP, Steal Thunder operates similarly but is useless in this combat system because you can get incredibly high IP, the deck system doesn't allow for it, and it doesn't work perfectly anyways. These are just some examples of skills from legacy that were incredibly well done, it'd of course be great to see new ones thought out too.
Going back to a single attack bar system where the attack to defense weight needs to be decently high to deal good damage would be a much better experience than getting kito'd twice, flexed twice, and cleaved for a third of your health bar. It's easier to defend with a single defense bar and the opportunity for skills that impact your opponents attack bar are more available. It's also incredibly straight forward resulting in a lower barrier of entry/understanding for more casual players. You can say with absolute certainty that no one aside from PVPers understands the impact/damage formula for multiple color attacks.
Make it so IP isn't as hard to initially gain and that spamming defenses doesn't just make it even harder to fight back after due to giving them IP. I'm not sure much else needs to be done for this aside from maybe removing defense spamming giving IP and allowing THINK to actually be used in combat (impossible now with deck restrictions).
It should also be noted a single attack/defense bar system allows for other weapons to be viable. In legacy we saw way more people using a sword/shield than we did B12s. This is because it was more reasonable to get a hit on someone with their defense at 0 then it is to get a hit on someone with 100% openings. The scaling was also better so hitting with 10 attack at 2 defense or something still did quite a lot. I think the scaling comparison with openings now doesn't work well for this.
Things the current system does well:
The Maneuver selection is decent and they work kind of well. It'd be cooler if you could use a wider variety of them without decks limiting you. I think shield up needs a nerf as of the last world changes though. The overall system is OKAY, but there are so many rooms for improvements and it could just be way better. I know you guys hate going back to old systems or scrapping new systems/concepts but if you had something that was near perfect and have had difficulty implementing something better since it would be for the best to reimplement what was almost perfect with a few changes.
TLDR:
Equalize number differences by implementing weighted attack/damage based on the differences between the number of people on each side of the fight. Introduce more in-depth skills (even if it's just re-adding the cool legacy skills that were removed. Remove or increase the size of decks to allow for more variety/opportunity to act on situational openings. Make it so IP costs don't completely punish the people who are outnumbered or getting attacked. Basically everything is best achieved with a single attack/defense bar (IMO).
TLDR of TLDR:
Essentially reimplement legacy attack/defense/skills while adding more skills, and weight damage/attack weight on the number difference of people you're fighting.