Non-damaging spells like Fear, Slow and Haste increase the target count when heightened. However, not linearly as Hold Person or Blindness does in DnD 5e (+1 target / level) but quite randomly:
- Fear gets 4 more targets for 2 more levels
- Haste gets 5 more targets for 4 more levels
- Slow gets 9 more targets for 3 more levels
Would this house-rule terribly upset game balance?
Every spell that has a heightened version that effects more targets, can be also heightened with the +1 target / level formula. You can still use the old formula if you wish.
- Haste for 3 targets: 5th level spell
- Fear for 2 targets: 2nd level spell
- 6th level Slow would still effect 10 targets, as the new formula gave you a smaller number (4 targets)
While this gives more flexibility to non-blaster Sorcerers, blasters already have it.
Table for Fear:
- first column (slot) is the level of the slot
- second (original) the number of targets with the official rules
- third (new) the number of targets with the house-rule
- fourth (better) what a caster with some common sense would chose