If you just want to look at numbers, the Black Friday Stingrays have already appreciated. They've sold for $200 or more, all for a bike that was $88 the first day and $50 or less by the time it hit clearance in February...
I've still got the box for mine, and the $88 price tag.... you know, just in case anyone's interested in paying me $200 for it...
I do think the interesting bikes of all types will retain good value, but it's hard to predict a "classic" or collector model. Anything that was made in small quantities is a good bet. Anything original and American is a good bet. Today's bikes? If it's sold in a regular department store, it'll be a while before it achieves any appreciation.
Current bikes that spring to mind as collectibles include the 1995 repro Schwinn Phantoms, the entire "Select Series" Stingray line from the repop 1998 Krates through the 2005-2006 Grey Ghost, the BFK Stingrays, the repro Columbias and Roadmasters that were done in the 1990s, the Felt cruiser models, the Schwinn Cruiser 7 or aluminum Cruiser models.
Will anything else be a "collectible" ? Only time will tell. Will it appeal beyond this current audience? Not likely. Will examples start changing hands at higher-than-original-list price? Not for a really long time.
My opinions...
--Rob