Didn't we datamine a second set of armor for each class way back when VoG first came out? Just use the nightfall idea from below, and make the checkpoints drop this new armor at the new levels, with a little extra goodies on completion as it has extra modifiers active.
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Remember that data-mining isn't accurate but I like the nightfall idea :)
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It doesn't have to be an accurate depiction of what will actually happen in game, just that it shows it exists/existed at one point. The models/names are already made, they just have to update them and bam new content people would actually play.