I've been consistently completing at least up until Lv.25 in the Series pass (more in Series 1) so I've been constantly gathering trophy crystals and effectively being "forced" to buy things I don't necessarily want just as a means of being able to get the next tier unlocked without overcapping.
I bought all of the unique rewards, and bought multiple Framer Kits for the jobs I play even when I don't even mess with framer kits. I've bought multiple Wolf Collars just to use up crystals, don't particularly care for most of those weapons.
I've bought the full recolor AF set for my main, I've bought a bunch of the recolor LotA and High Allagan recolors pieces.
All of these things I don't particularly care for but are just adding to my inventory space, unless I dump them into other stuff like the Wolf Collars or just buying Framer Kits I don't care about. I could dump them into Trophy Crystals -> Wolf Collars -> Wolf Marks -> whatever but that seems incredibly wasteful considering I'm already in the same situation with needing to overspend on Wolf Marks.
I can provide TWO solutions to this "problem":
1) EASY: Increase the Trophy Crystal cap.
--- Grand Company Seals are able to go up to 90,000 which would be a great start.
--- Island Sanctuary goes at least up to 999,999. This would for all intents and purposes likely solve the problem outright for 99% of people.
2) MEDIUM / HARD? (Easy...): Add more unique rewards.
--- There are 13 MOUNTS and 12 OUTFITS from previous Feast rewards that could be incorporated.
--- A regular Trophy Crystal set is 7,500 crystals. If the Feast rewards are viewed with more prestige or want to be more of a crystal sink they could be something like 10,000 and given in a single boxed set (to save inventory space while earning). In total that'd be 120,000 as is, which would be a lot.
--- Alternatively, the highest Wolf Mark set is currently 14,000 Wolf Marks, so you could just do the same with Trophy Crystals for the old Feast rewards. (168,000)
--- Old Feast mounts could easily be set at 15,000 (195,000) or even 20,000 (260,000) Trophy Crystals as a great sink. This would be similar to the Wolf Marks mount being 20,000. These would last for quite a while as things to buy going forward into the future.
--- The old Feast rewards would take no additional work to implement other than whatever is necessary to incorporate them into the vendor system. Which I'm sure is a lot different from making new stuff from scratch.
--- Create entirely new unique rewards to get per Season / Series...
--- I guess more recolors...
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As it is now I'm dumping Trophy Crystals into things I don't particularly care for anymore and it doesn't feel great. I'm up to Series Level 7 on the new Series pass and have not bothered accepting my rewards because I'm capped out and don't feel like buying anything else...