Trigger doublers are already a good start, and theres lots of potential subthemes to explore with Isshin, like myriad, mobilize, or firebending, to name a few mechanics
The rarest misprints are often the cards that take pokemon investing to the next level
For example, keyword searching only turned up one book card in the 1998 Urzas Saga set, but in a visual review I found 19 cards that feature books
Card Stock: Authentic cards use a specialized layered paper (often called "Blue Core") to prevent light from shining through
Artwork has levelled up too: textured foils, unique print patterns, and full-art treatments that make cards feel like mini posters
Foil cards will weigh more than their non-foil variants and, when weighing, make sure you are comparing like for like