I do think PvP returning to Maple is a welcome addition. The main counter argument I hear from this is that PvP was frustrating to play and it wasn't fun. Although it is true that the class balance wasn't great, it also wasn't terrible. At least 50% of the classes were viable if you were aiming for the highest possible score, and really the only class that was garbage and had no potential was Wild Hunter. Now of course if you enter PvP thinking that you can get away with attacking as if you are grinding/bossing, then you will get destroyed by players who know what they are doing. So it's not so much of the system being flawed, it's just people don't bother with changing their playstyle and chalk it up to "lag" or "flawed system" when they get completely outplayed.
For balancing gear, the old PvP system did this perfectly by assigning everyone the same arbitrary base number(no matter what the gear was), and the only multiplier was the skill % and Battle attack, which at max only raised about 5% of your damage. Next to no effort needs to be put into this, as being able to ignore the math of your equipment should be less work for Nexon, and plus they already have a blueprint on how this should work with the old PvP.
Skill balancing shouldn't be a huge issue as well. The skills were relatively balanced in the old PvP, and some of the more stronger moves in training had cooldowns attached to it in PvP, which not only helped balance damage, but also added a dynamic playstyle for these classes. Most mobility moves other than Flash Jump type effects were banned, so characters won't be flying around the map and will involve more decision making.
As for the rewards, they can make it more about the glory and satisfaction rather than just having more stuff. Titles, Chairs and other cosmetics can be awarded to players who achieve certain points or playtime in PvP. Of course we should still have rewards that help outside of PvP, but by making the main reward have little to no advantage in training/bossing makes hackers have no incentive of ruining matches. It should be about the satisfaction and bragging rights, not necessary the reward.
For my suggestions for "New" PvP, it needs to be cross world. Old PvP did have a community in the five major worlds, but other than that from what I know it was pretty dead. Near the end of PvP one of the players in Scania had to buy over 30 World Transfers just to set up a small tourney that featured some of the best players from each World. This shouldn't be necessary and players from different world should be able to queue up into each other. Speaking of tourney, there also needs to be private lobbies to host tournaments and play custom matches. Auto queuing is fine, but it's hard to play against friends and set up tournaments with just this system. Respawn needs a rework as well, and the easiest fix is to completely remove the safe area and just give them spawn shield for 3 or so seconds. I am probably still forgetting a lot of stuff, but I don't want to ramble any longer so I'll leave it at this.