The point I want to make here is offering suggestions that will add motivations for players to grind, instead of just old-school "grind a lot for little rewards" that discourage players from grinding. Of course, the game is cool at not asking players to not too focused on grinding and let them taking break, but I think it is reasonable to offer solutions for people who actually want to grind, while players who take break can still chime in occasionally and get some well-earned rewards for their contributions toward the liveliness of the content.
Back to the topic, after you get 28k Crystals from the series milestone and reach lv30, it will require 20k EXP for a mere 1k crystals. It doesn't feel like a grind many players want to do since too much efforts for little rewards.
For anyone interests in getting Hellhound stuffs or perhaps the reskinned Virtu gear for whatever reason, this becomes super grindy as these items require a large amount of crystal (10k crystal/hellhound weapon). In case you just want to buy all current one-time items (job actions and barding/minion), it requires 30,500 crystals, so you will have to grind 60k EXP to get them all.
I feel like challenge logs should update to provide trophy crystals, so that people will have motivation to pvp for weekly goals. Perhaps 1,000 per "participation x times" goal and 1,500 (or 2,000) per "win x times" goal.
Daily frontline should net you some crystal too. Prefer 500 crystals per pop.
Additionally, allowing to trade wolf marks into Crystals. 20k mark should yield you around 5k crystal (4:1 ratio).
Also, to prevent players hoarding crystals and buy off new rewards of next season day one, next series should use a different type of currency and convert the outdated one into wolf mark, not so different from how tomestone works.
In addition, series milestones should have crystal rewards scaled properly. If I grind 2,000 exp for 1,000 crystal, I believe grinding 5,000 exp should give more crystals sound reasonable enough.