My bets on future collectability are as follows:
Kestrel's early monocoque bikes, these were there in the beginning of the era.
Lugged carbon bikes, everything from relatively common Epics to Colnago c-59's.
Calfee's bikes, USA-made and with their unique build style (I own Carbonframes
#20 ).
Bikes/frames that were the first to feature (or popularize) a particular feature, such as a tapered steerer, extended seat tube or modern disc brakes.
1st-year models with identifiable functional or cosmetic features which led to marketing success (Roubaix/Zertz for example)
Frames/bikes from companies that are small, but which go on to become highly successful.
Bikes that are particularly good-riding tend to be highly regarded, and also tend to get "used up" so become rarer to find in top condition.
1st 650B production mtb. Good luck naming it. This wheel size may take over!
Limited-production bikes (which of course tend to be of upper-end quality), rarity counts.
Bikes on which a top racing team was particularly successful, or on which a particular sprinter, climber, etc. was particularly successful/famous.
Over the last 65 years, we've moved toward bikes that are a bit lighter, have a greater number of gears, have integrated shifters and slightly more advanced everything else.
Dedicated self-powered bikes may become a much-smaller share of the market, sold as either sports competition models or as entry-level transportation, while bikes equipped to accept some degree of electric assist may become predominant, with suspension to handle higher speeds.
The first few versions of electric-shifted bikes were cool, but not as cool as the first bikes to completely hide all brake hoses and wiring.
I can also see bikes with on-the-fly cockpit adjustability perhaps becoming more common, as well as safety features that likely would seem extravagant from today's perspective.
One thing for sure is that as long as I'm wrenching and riding, I'll be keeping an eye out for good newer bikes with severely-depreciated used-bike prices attached.
The 1997 Calfee and 2004 Orbea I've bought recently are perfect examples.
These bikes both have extremely strong frames/forks that are not going to go "soft" in my lifetime.
This 2015 LOOK would be a really nice garage-sale find in 20years. Looks like an attacking cat/animal!
I had to wait 30 years to score this one for a good price:
And this one was had cheap after only about 12 years of depreciation:
