Honestly, interdimensional travel often feels like a cheap way to generate stakes, but I think the best conflicts come from the emotional toll. Characters get back to their own dimension, but they're fundamentally changed. They've seen things, made connections, maybe even fallen in love with someone in a world they can't stay in. The 'what if' and survivor's guilt become the central conflict, not the big bad monster. The dimensional travel itself is just the catalyst for a more intimate, human story of loss and impossible choice.
It also creates a brilliant source of political or ideological friction. Suddenly, two societies with completely different histories, ethics, and technologies are forced to interact. One dimension might view magic as a sacred gift, another as a mere utility to be industrialized. That clash of worldviews—colonialism, resource exploitation, cultural erasure—feels far more grounded and terrifying than any generic portal monster. The real villain isn't a person; it's the inevitable conflict of two worlds colliding.