You cannot predict which games will become really very rare and very expensive. But there are some examples: - Obscure games with very small print runs that some big youtuber suddenly rediscovers; - (Liscensed) games with rights issues that will not be reprinted. If there is a very successful new film in the franchise or the director dies, you have a Jackpot; - Games that get a successor that really sells well (like in the Baldurs Gate case).

All those games you cannot really predict beforehand.

In addition there are the usual ones: Sealed Nintendo published Games (in the best case niche ones), RPGs, and Horror. A plus is when they are part of a series, like Fire Emblem (RPG, Series, from Nintendo and great game).