Would DnD 5e style heightening unbalance the game?

Question

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.

Examples:

• 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.

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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

slot	original	new	better
1	1	1	1
2	1	2	2
3	5	3	5
4	5	4	5
5	5	5	5
6	5	6	6
7	5	7	7
8	5	8	8
9	5	9	9

Answer 1

Probably Too Strong

This, in some ways, is similar to combining spells. A 2nd level Fear spell would be the same as two 1st level Fear spells; similarly, a 4th level Haste spell would be the same as two 3rd level Haste spells, without even considering the action consolidation. The closest comparison I can think of that we have is the Spell Blending Arcane Thesis. The Spell Blending Thesis states:

you can trade two spell slots of the same level for a bonus spell slot of up to 2 levels higher than the traded spell slots.

So, from this, we can make the rough comparison that two spells of the same level are roughly equivalent to one spell that is two levels higher. So your 4th level Haste would be (roughly) equivalent in power to a 5th level spell. Obviously, this isn't a perfect comparison (i.e. 3rd level Fear hits 5 targets), but it might be as good as one as we can get RAW.

Is Too Strong A Problem?

Being "too strong" might not be a problem for your table. Just make sure all your players are aware of the (potentially significant) buff to casters, and are also aware that enemy casters can receive the same buff. It might not be balanced, but not all tables want balanced play, and that's okay. I do, so I might be a bit biased :)

Secrets of Magic

Secrets of Magic is going to have a Flexible Casting class archetype in it (I believe this info can be found in a recent Paizo stream, though I'm not sure which...). This will probably get some of that 5e like heightening that you want. It releases this summer, in August or July I think. Maybe June.

Answer 2

As always, adding more options never makes the characters that are able to use them less powerful.

Haste and Slow spells are pretty good in PF2, both in their heightened and non-heightened forms, so you can expect most savvy players to already use them. (I don't like lvl 1 Fear because I find the frighten effect way overrated, but some like it, anyway the heightened one is an efficient way to get rid of multiple trash mobs when a lvl 3 slot becomes cheap). The houserule you offer won't suddenly make players consider using underplayed spells.

One problem you may encounter is rather that some player may start only using those spells, which is quite boring if you play a long campaign. When my high-level caster prepares for a boss fight it is not unusual for her to prepare Slow in a 4th-level slot because all the 3rd-level ones were already taken by other Slow and Haste spells: with your houserule I would feel encouraged to do that even more.

Speaking about raw power level I think what you have to fear is lvl4 Haste and Slow. Lvl4 Haste would be a two-actions investment that immediately pays for itself if you target the right team members (compared to lvl3 Haste which needs two turns to become action-efficient). Sure, lvl7 Haste targets 6 allies, but usually you won't have 6 interesting allies to target so this is not as big as it sounds. In the same way, lvl4 Slow ensures that the bad guys loose at least as many actions as you (unless they critically success): good deal! Maybe not as good as the lvl6 one, but encounters that feature that many opponents to target are not that common.

As a remark, I am not sure why you would want this houserule in the first place.

The heightening system doesn't really give more flexibility to Sorcerers in PF2, even the blaster ones, as they have to separately "learn" all variations of their spells for each level they want to heighten them. Also being able to make a bigger fireball doesn't offer as much versatility as being able to make a spell multi-target.