Having watched the "collectability" tide rise and fall a few times, I can tell you that there are three different types.
First type is the FAD, when something suddenly becomes popular, leaving the manufacturer as surprised as anyone (and stocks low). The response is usually for the mfr to boost production and try to stay on the wave as long as possible, while knock-off artists flood the market. Examples include everything from fidget spinners to POGs to Beanie Babies to the 1978 flood of "Star"-this and "Space"-that, to try to take advantage of the Star Wars boom. Some of these products may be collectable again a few years later -- often because they were so popular that nobody bothered to save them.
Second is the TRADITION. Disney and Star Wars stuff are the biggest examples. Year after year, decade after decade, certain items in the field see interest come and go, and with that will follow the prices. PEZ dispensers are the BEST example, designed and marketed to be collectable. If you get, say, 5 of every one that comes out and hang onto them a while, eventually you will make a profit selling four and still get to keep one, simply because so many turn out to be for characters that don't last (such as the Green Hornet) -- those are worth more than the ones which stay in production for a longer period.
Third is the MEMORY. Got an original Corgi James Bond Astin-Martin? A big Fireball XL-5, complete with figurines? Monkey Patrol gun and helmet? Johnny Action (One-Man-Army) set? Stuff that has been long out of production, but is fondly recalled by people who care enough to try to find them. They weren't collectable then, but there aren't a lot left, so it's a seller's market.
What you need to watch out for is "Franklin Mint Syndrome," where you buy every Special Edition or Commemorative Series that comes along. You're best off staying with what you already collect yourself, and what you KNOW.
This reminds me that there is also what could almost be considered a FOURTH kind of collectable -- ENVIRONMENT. This is practiced by people who focus on a particular period of history, and try to recreate it by getting the things which were found at the time. Go into their homes and you step back in time to whatever year they chose (in some cases, each room has a different theme). I know one woman who has paid for her hobby by passing her name to WhollyOdd -- er, I mean HOLLYWOOD set dressers. If you are doing a film set in, say, 1973 (she does several different eras), not only does she have everything found in a home back then, she can tell you exactly how it would look in a "lived-in" house, or a "clean-freak" home, down to Flintstone jelly-jar glasses (which would, of course, have been around for a decade or more, since the kids were little) and Best Foods (west of the Mississippi) or Hellman's (east of the Mississippi) mayonnaise jars, along with other food packages of the era. She walked on one set, stopped and told the set dresser that some particular toy on the floor wouldn't hit the market until a couple of months after the date on the calendar that was on the wall. She is far from the only person like this out there -- set and prop guys keep the names of their experts secret!
I contrast that with some prop and set guys I've seen who figure that if they're within 20 years one way or the other, nobody will notice.
So, the bottom line is that nobody can predict. If you are looking at this as an investment, FORGET IT. Collectable value isn't something that you plan, it's something that you DISCOVER.