I can't think of anything, either rare, ancient, or extinct that I have a hankering for. I honestly think alot of the old/rare feathers, were used simply because they were pretty, and available at the time; not for any particular fish-charming properties. Indeed, I think some of them were sought and used purely
because they were rare.... bit of a kudos thing
I'm more than happy with what's available today. Even my humble collection of fur and feather will keep me catching fish and give me scope to both replenish and experiment for a thousand years.
There's some stuff that there's simply no substitute for... Jungle Cock for example, and seals fur. I hope those are available for many a moon.
Blue Jay is one thing I'm happy to sub these days... there are those who say 'no way', but the more I think about, the less critical I think actual Jay is. Sure, it's a lovely feather and on the right fly gives a great effect, but I don't think it's irreplaceable. Theres some lovely hen around these days, in some interesting shades of blue. Also - Guniea fowl, Partridge, etc.