Increasing Monster Density is point 3 on my list of needed improvements for Terror Zones.
Specifically I think Terror Zones should have Guest Monster packs that can spawn in Terror Zones. That would be a way to increase monster density, but also allow some flavor making the Terror Zone different from the regular area. It would also allow more fine tuning of Monster Density. A small Terror Zone like Tristam would only have 1-2 Terror Monster packs that could spawn in the Zone, but a larger area like Tal Rashas Tombs might have 4-6 Terror Monster packs that can spawn.