I'll drop my 2 cents as a recently returned player:
PVP
GP: Reset them more often and consider wigling the numbers. Bastion giving 100 is (kind of) the core feature of Aion 4.6 but may be overtuned.
Arenas are broken, it's basically an unplayable mode except for the top 5% of the population
Penalize AFKing dredgions: Contributing with DPS, healing and other activities should give you the points while staying in base doing nothing should give the bare minimum. Currently winning gives aprox. 13 000 AP vs 8000 losing, and the gap between those that actively take part and those that don't is laughable.
Sieges: TBH the current system is pretty fucked up so I'm not sure about what could be done to fix it overall, but at least forts should become balaur after a number of successful defenses (2 or 1?) to encourage rotation.
Equipment is an issue and I don't know how to fix it but I'll flag it nonetheless. Currently any new player is just fodder for the first 5 months of the game as every tryhard has +15 doublemanastoned amazingness in every slot while newbies take even several weeks just to have an unenchanted, unmanastoned BM set.
PVE
Make more late-game instances relevant (looking at all of those entrances in the Underground I have never set a foot in) to encourage rotation. This can only be done through changing the loot these give into something relevant. Accesories could drop from more than Sauro (or maybe Sauro gives 3 parts and another one gives 2 parts, you know the deal) while others give manastones, skins/enchant stones, BM and other generally valuable resources as an alternative to solo-farming (remember guys, this is an MMO after all, so it's supposed to be social too!)
General
Balance between classes is non-existent. I don't have enough data or knowledge to tackle it in depth, it's more a (generalised) feeling.
And also: bring the new visuals to Poeta. Does it add anything? Not really, but it's very pretty.