During the last week (or 2) you should have all legacy challenges/collectibles active. By this I mean all Leviathan/SotP/CoS challenges and legacy nightfalls. This then opens the accompanying collectables to be obtainable.
As much as many would love it, I am not proposing multiweek quests in 1 week or all the dreaming city queen's lore, as that is not in the same spirit as how all GM NFs are available near the end of a season before they go away. This is similar due to this stuff going away into the DCV.
I know challenges/collectibles are not completely the correct words, but they should get the point across.
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It would be pretty cool to have a final week where everything is completable but I'm not betting on it happening.