1) Make cancel return buffs
2) Reduce the duration of all debuffs. Debuffs like Paralyzes, Stuns should last max 2 seconds. Debuffs like roots should last 3 seconds, debuffs like hex, slows should like about 10 seconds.
3) Reduce the critical damage of auto attacks for ALL classes. In retail l2 the game plays around critical dmg due to buffs+items+skills. If you do this it will have as a result many fighter skills becoming uselFUL, skills like Judgement and even low skills like Power Strike (if boosted a little bit). So the game will be a lot more interesting instead of mainly pressing F1 for dmg.
4) Reduce the overall buff slots (when I say overall I mean buffs+d+s+everything else) to 22 or max 26. Why do I suggest this? Because with limited buff slots build options for each class can start becoming a thing. For example, one Dark Avenger may choose to go to fights with offensive buffs, while another DA with defensive buffs. Some other DA may choose the buffs for his choice for his own reasons.
So to conclude this will make 1v1 but EVEN MORE mass pvps A LOT more interesting, a lot more diverse, since facing 2 DAs may simply not be the same deal at all! This will take mass pvp to another lvl.
5) Self buffs should not take buff slot. Because there are classes with a few self buffs and others with many self buffs.
Also, some classes use more buff slots than others. Classes like Shillien Templar and Dark Avenger use 3 type of buffs. Tank buffs, Mage buffs and Fighter buffs which means they should get +2 extra buff slots.
Mages use 2 type of buffs, Mage buffs and Shield buffs, which means they should get +1 extra buff slot.
And lastly classes like Gladies, Tyrants, Daggers, Archers etc use only Fighter buffs, which means they should NOT get any extra buff slot.