It would be cool if we had class selections that opened or closed the skill trees. Then New Game+ could be implemented at 2500, people could try new classes(and hopefully new buffs) that weren't in the first round of content. I've always liked that about RPGs and it would be a welcomed addition because different classes and races might make the game tougher or easier.
Official jobs/roles where you get exp based on how much you use that aspect and the more rewards/enhancements you gain from experience. Potion makers get rewarded with higher success rates and lowered components needed. Farmers get higher drop rates with a new enhancement named Scavenger or something. Composers get shorter brewing times and more exp. PVPers get higher thievery rates and chances to negate the dreaded 2% miss and Piercing Strike. GVGers could get stam saved from battles and an increased GVG target range. Arena masters could get lowered fees and bonus rewards. Titan hunters could get a lightfoot enhancement and increased stats against titans.
Gives incentive for people to try other aspects or to specialize in a certain one. More ways for lower levels to gain FSP if they choose to specialize in one aspect(similar to plant farmers now).
Never coded a game before but I don't think it would be hard to do something like this. Composing EXP formulas could probably be used as the basis then just tie in the EXP earned to new enhancements that can only grow through experience and not buffs.
I'm in, sign me up.

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