The best gear is too strong, people in decent shells are overwhelmingly powerful, the content has not changed to take this new gear into account.
This means the top players have nothing to do because they beat everything and all the content you have left is too easy for them to repeat.
These players will quit because there is nothing to do and the game is easy.
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This also means the players without that top gear (that the game is now designed to take into account) and without good shells find the game way too hard and can't do anything but ambuscade.
This means these players can't do anything and are forced to pay mercs, they will also quit because they can't do the content.
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You made the game too easy for hardcore players and too hard for casuals, the amazing double whammy.
Some suggestions before my sub expires:
Add an ultimate mode to all current content over time (omen, ambu, dynad etc), which is far more difficult but more rewarding (add titles for clearing these). Should also add unique new armors with NEW TEXTURES that drop in these modes with a rare chance to encourage them to do them (they don't have to have super power stats). Alternatively you could add a way to change any REMA to have a black glow or something by doing them a set number of times. Basically, you want to do something to make sure your best players want to do Ultimate mode clears that isn't some OP drop that makes ult easy too 6 months from now.
Add +3 ambuscade upgrades to lift the lower players up a bit more (make them long-term goals is fine), also add more -dmg on them because casual players are far less likely to have -dmg sets. This will allow them to do more lower tier content.
Possibly add new ranged ammo gear that gives other stats boosts to trusts for solo players attempting harder content, maybe even -dmg to the trusts. Add this to Odysey or something so people wanna go do your new stuff. If you want casual players to progress from ambu to odyessey you need a good reason.
Greatly increase the amount of HP WKR instances (just increase the HP, don't make them stupid difficult or you will ruin it) have and offer rewards to players who do them in terms of larger amounts of CP, so they can make progress on job points by doing these WKR fights as a community. A player who can login and do WKR with others as a world event (boss and side monsters) and get decent CP this way, and so won't pay mercs for it. Even really casual players can come in and just kill side mobs for CP gain. It should be below a proper apex party but still better than other options (if it's not good, nobody will do it). Due to the increased HP and possibly -dmg abilities if the HP bar drops too fast the fights would last longer, people could make WKR chains and go from one to the next. You could also add RoE objectives where you have to kill them 100-200 times to get specific rewards (for better geared players to do it more), maybe cosmetic maybe mounts, maybe something else. (also make all the WKR gear 119 and put some of the Delve rewards into WKR as RARE drops, people do delve for other reason than the gear anyway)
While on the subject, make all Ilevel gear into 119. You punish players Trust level when it's below 119. There is no reason for sub 119 ilevel anymore, it's just useless gear in the database right now and a lot of players don't know ilevel = trust level.
Also increase the reward from the Ambuscade v2 DAILY if done alone in terms of an additional 50-100 hallmarks, and increase it more for every additional person you take into the instance (only on the daily upto the level of a v1 reward). This would a) encourage players who can only do v2 to GROUP UP and possibly make friends and join shells, make daily v2 groups a thing and lower queue times (cause they group up instead of solo). Due to this increase only applying to the daily, it won't impact much of anything else but make them feel like they are making better progress. You would see shouts like "lfg to do v2 daily!"
The random NM idea (that gives +100 marks) in Ambu v2 should also stay forever and not just this bomb one. Maybe even add it to V1Ve runs.
Also in relation to Lillith BCNM (VE to E), allow any armor you already have that drops from her instead give a dark armor shard that stacks to 12(only armors, not weapons). You could then trade 5-10 dark shards to an NPC to get pieces you don't have. This would make it more rewarding but still keep it as a long term goal to complete a set, it stops players giving up trying since the more drops you get the more chance of getting 0 drop.