Serious answer now, my predictions are:
- what we know has retro now will not be elusive to the new generation, perhaps the few that are seeking vintage but will have no experience of the quality or experience in the real world, they will be driven on documented myths from the internet.
- Garage queens will gain value for a very limited market only. Fashion may play a part in X years from now. Personally, I'm more interested in solid riders so I'm out of that game.
- Low value MTB stuff will go in the bin. Compare this to the fixie trend, there is no market for the heaps of low value road tat from the 80s - it must be Campag or Dura-Ace or Mavic or Modolo exotica.
- short travel suspension forks (< 63mm) will dry up rendering most quality frames almost useless except for the fanatic.
- mid travel suspension forks (with disktabs 80mm) will dry up rendering most quality frames from the late 90s early 00s useless. The generation who made the transition to 120mm will be old and not be able or stupid enough to do jumps and will long for a short travel quality fork. I've have some NOS stockpiles for my own builds future builds. They are already scarce.
- 9 speed will die a natural death. Prices have fallen but are maintained due to people squeezing life out of their old set-ups, but they will rise again. I've stocked up on full transmission parts till I'm 70 for 6 bikes
- 25.4mm stems and bars I'm always stocking up now while they are considered obsolete.
- square taper.....I've seriously stockpiled on NOS UN72. Actually, I've purchased every NOS (in the right dimensions) from the bay for the last 3 years.
- v-brakes. Parellel link type from Shimano will be sought after. Look at the bids already.
- Front mech with no plastic shims in the proper band diameter - TP and BP. I've stockpiled here since they are already a thing of the past.
- Ceramic quality rims and brake pads will be sought after for the weight weenies riders of tomorrow. Considered obsolete today.
- all Carbon will rot in hell. Too dubious for riders and will be confined to the garage queen or adventurous old age pootlers unless it it NOS.