Extra ToH takes the most work for certain... hence why Namin? did not suggest it. The GMs have already stated that even the monthly ToH is already difficult to maintain, because of its length. Not everyone can benefit either.
Raised rate is relatively easiest... existing, non-custom aesthetic headgears is most ideal, but Namin? believes that there are no aesthetic headgears left because they are all added in the Hatter NPC's list of possibility already if they do exist. Rates affect everyone regardless of time zone, as long as they play. It requires a reboot before, and another one after just like the celebration day/week/however long it is.
Can always adjust the actual boosted rate or duration if abuse is a concern.
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OCA's only problem is perhaps distribution... 1 OCA per account may result in people making accounts to get OCA. Adding a level requirement will bar out some players, which goes against the original intent of the small reward. Figuring out these technical issue may end up to be a headache...
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What Namin? personally wish is just a good old fashion poring smash ._.; heRO never had one of those since new svn is released... but then, this goes against how the accessibility rule.
Namin? miss poring smash...