The vintage aluminum bikes seem to take a beating in the used market. It is hard to say whether they will bounce back. There is a lot of stigma against them.
I think there will be a continuing market for steel frame bikes. I'm partial to lugged steel (mostly pre-90's).
But the TIG steel and unicrown bikes from the late 80's and 90's are good solid bikes, and may enjoy an increasing market.
As far as rarity, it is hard to say. One thing that seems to be happening now is that with a reasonably strong US economy and favorable exchange rates, the Europeans are dumping quite a few bikes onto the US market. That may continue, or could change at any time. Perhaps we'll see a reverse flow of bikes, with the Italians snapping up vintage Colnagos from the USA.
The strength of the MTB market from about 1980 to 2000 or 2010 has surpressed a lot of the 80's. 90's, and early 2000's road bikes. I'm actually a bit surprised with the number of that era of road bikes showing up on the used market. But, it may benefit the vintage market somewhat.
I'm also partial to the European or American bikes, but there seems to be a good healthy market for that Japanese bikes, although often not commanding a lot of money.