The thing is, the Competitive thing basically comes from itself, so the only thing you can do to influence it, is by adding variety in Game Modes, such as Open World PvP, Arena Matches, Duelling Rings or maybe even PvP Raids (basically contest Dungeons from other Players, some of the early MMOs had this), to name a few.
Now the complexity, is the entire lifeblood of PvP. With complexity, i mean how the Classes in the game are designed, how they work with the enviroment/other Players and if the possibility of alternative Playstyles exists. Now what exactly does that mean? It means that your Skills come with effects that have impact on combat without just dealing damage such as Slow effects, Stuns, Knockback Abilities, (De-)buffs, Skills that allow the Character to jump fixed distances instantly/delayed or straight up manipulate enemies/allies (for example, mind control from WoW or rescue from FFXIV), to name a handful. Those kind of skills are what allow Classes to have a certain Playstyle that is both responsive and dynamic and allows even specialist Jobs such as Healers, to actually put up a interesting and good fight despite damage not being its main thing.
Now with that Info being kept in mind, keep a look at FFXIV PvP. It has almost nothing of those kind of things at all. Most abilities are just raw healing/damaging abilities that sometimes trigger under specific conditions, with the assumption to keep bursting in check. To make matters worse, they even gave every Job Combat Potions which unnecessarily stall combat for way longer than it should, in a 1vs1 scenario.
If this game wants to have a functioning PvP enviroment, it needs to keep those things in mind, as well as design Jobs about individual performances aganist the other Jobs, in a way where the possibility of being able to defeat any of the other Jobs in a 1vs1 scenario exists. Anything else, such as Group Fights/Feast and Frontline's will automatically revolve arround those design choices on their own, so worrying about how specific Jobs work inside a Group enviroment comes second to individual performance (that does not mean it isnt important, since this too, is a important aspect (the Shai Class from Black Desert Online is a good example on that, as a form of reference) that can be the playstyle of a specific Job.