My gut instinct leans toward 'brood' as the heaviest, most suspense-friendly synonym for ponder. It carries this slow, simmering quality — not just thinking, but thinking with a dark temperature. When a character broods, you're not being handed tidy conclusions; you're being led into a fog where every stray detail feels charged. That fits suspense because the whole point is to delay resolution and let tension accumulate.
Compared to other options, 'ruminate' is more clinical and reflective, 'mull' is casual and breezier, and 'speculate' sounds more outward-facing and speculative. 'Brood' implies an inward storm: the mind turning unpleasant possibilities over and over, like a film score tightening under the surface. I often swap in lines like, "He brooded over the empty chair," instead of "He pondered the empty chair," when I want unease.
If you’re writing a scene, pair 'brood' with sensory detail and pacing. Short sentences, interior fragments, and physical manifestations — clenched jaw, insomnia, fingers tracing old photographs — amplify that brooding state. It gives readers a slow-burn dread that feels inevitable rather than contrived. In my own drafts, choosing 'brood' has often shifted a scene from merely thoughtful to deliciously tense, and I love that little nudge it gives the reader's spine.