I completely understand. In terms of visual stimuli, it could be something simple like changing the model of the ghost or maybe enhancing the sound it makes when it jumps you. It might be an angle you could take. Rather than change the appearance, make the build up more intense. As suggested, adding like more subtle sounds as they approach, maybe tweak the detection audio. Though I fear if the build up to the scare is more intense than the scare, it could downplay the actual ghosts themselves. So tread this with some caution.
STILL, this is amazing work, I will definitely try your other game(s) when able. Frankly if you made a boxy low poly looking game that made me jump my pants more than a lot of Horror games with astronomical budget that I got bored of within 10 minutes, you've got a formula on your hands. Please keep doin your best and hope to see more.