Designing non-KMS must be a challenge (TMS doesn't really play by the rules) is a real challenge. Some content like Blackgate City offered mediocre accessories and armor that fail in compared to the items dropped by bosses scatter throughout Maple World while some like Gollux is really strong (despite non-KMS items getting lower tier flames.) If you make rewards from content awful, it would be a waste of an attraction, but making it too strong is of course, too strong. Gollux is mess not because of the abundance of belts and pendants from farming, but rather the rings and earrings being in stock once at the shop. What happens if you boom yours?
To promote more usage out of bosses, the rewards from the boss need to appeal to players. Arkarium gets a lot of traffic in KMS due to the Dominator Pendant being the ultimate pendant in most setups (the second pedant varies wildly because some might MEE a Rising Sun Pendant.) In GMS, Arkarium isn't used since the Reinforced Pendant and Superior Pendant easily overpower the Dominator Pendant. Pink Bean is already used since its the only pocket accessory you can obtain that isn't Hard Will's. Hilla and Hard Hilla are farmed for the Blackheart Pet in Reboot, but there isn't much appeal in regular servers. Magnus became relevant after Ghost Ship Badges being discontinued (and possibly Sengoku Badge.) KMS has a better reason to go after Hard Magnus since they don't have access to Gollux and the Dreamy Belt is stupidly rare. CRA and Absolab is self explanatory.
For the content from bosses GMS finds useless, the max reward for that content should be raised to provide an alternative to Sweetwater and Gollux while making use of the initial boss. By raising the level of the current boss accessories, those items will be subject to the gains available past 15 stars, thus making them a viable option. If a Noble Ifia Ring were raised from 120 to 140, it would have it star cap increased from 15 to 25 and be a possible substitute for a Reinforced Ring for it being the same level and being a boss set item (slight gains.) Also with the star car being higher, items would boom and provide a reason to revisit that boss (it boomed.) Redesigning existing equipment so they are as enticing as our broken favorites will allow for other alternatives for equipment and usage out of underused bosses.