In my humble opinion, I think Pathfinder 2.0 has incredible potential to address the power of spells and spellcasters. This is because of its strict action economy. Characters are bound to three Actions per turn as well as restrictions on those actions (multiple attack penalties, for example). Feats often exist to remove those limitations under specific circumstances. I'll give some examples from the Barbarian:
Sudden Charge (Barbarian 1): Spend two Actions to Stride twice and then make a melee Strike against an adjacent enemy.
Swipe (Barbarian 4): Spend two Actions to attack two adjacent enemies with one attack, essentially removing the Multiple Attack Penalty (MAP) for the second attack.
Furious Sprint (Barbarian 10): Spend two Actions to Stride four times, or three Actions to Stride six times. but only if you move in a straight line. Fun Fact, you can't use RAW use Sudden Leap with this ability to jump during this movement, which is strange.
Whirlwind Strike (Barbarian 16): Spend three Actions to Strike all adjacent enemies with no MAP.
So, as it stands, all characters interact with the Action economy and martial characters have ways of removing specific limitations under specific circumstances. Therefore, why not apply the same design to spellcasters? We've seen this with the Heal spell, it provides different healing and effects based on how many actions are spent casting it, and it's brilliant design. But I think we can go even further. Here is my proposed adjustment to both Feats and Spells, which works in tandem as you will see:
1) Feats are divided into 1st Level, 8th Level, and 14th Level. 1st Level Feats includes old level 1, 2, 4, and 6. 8th Level Feats include old level 8, 10, and 12th. 14th level Feats include old 14, 16, 18, and 20. This more clearly divides low, mid, and high level play, and lets the designers keep the 'gritty' stuff to 1st Level feats and the 'superheroic' stuff to 14th Level feats, with the 'mid-range' options at 8th Level. This also makes the system work better with the 'E6/E8' types of players and GMs.
2) For the sake of simplicity, the number of non-cantrip spells a spellcaster can know/prepare/memorize is roughly 1 + their Level in spellcasting classes. Maybe Wizards can add tons to their spellbook, or druids and clerics can access the whole list in theory, but the total number of spells they have during any given day is limited.
3) Spells are divided more clearly into Cantrips and Rituals, and the number of Cantrips is expanded. Cantrips are spells whose only cost is Actions, and are therefore limited to combat-oriented effects. Some higher-level offensive spells, like chain lightning or even fireball, can become cantrips with reduced base power Cantrips no longer scale with highest spell level, instead they rely on a caster's magical fuel. Rituals, meanwhile, require longer periods of time to cast than is feasible in combat, but cover all manner of healing, utility, and status inducing effects.
4) You can spend a Spell Slot to either (1) boost the power of a Cantrip or (2) cast a Ritual in combat. You spend one Spell Slot per Action involved in the casting of the spell. That's right, the more Actions the spell requires, the more magic you need to invest to empower it. If you spend multiple spell slots of different levels you use the average of their Levels, rounded down.
Example: Fireball is a Cantrip. You can spend three Actions to fire a 5-foot burst of flame that deals 1d6 + your spellcasting modifier damage to everyone inside the burst (Half on a successful save, none on a critical success, and double on a critical failure). You can spend three Spell Slots to increase the radius by 5 feet per Slot Level and the damage by 2d6 per Slot Level. Ergo, a Fireball cast with three 3rd level Slots would deal 5d6 + spellcasting modifier in a 20-foot radius.
Example: Heal is a 1st level Ritual. You can spend 10 minutes casting it to restore 1d8 + your spellcasting ability modifier hit points to a willing adjacent living target. If you spend one Spell Slot, you can cast it as an Action and they regain 2d8 additional Hit Points per Spell Level above 1st. If you spend two Spell Slots and two Actions, you can cast it on an ally within 30 feet instead. If you spend three Spell Slots and three Actions, you heal all living creatures within 30 feet but your healing total is halved (after rolling).
Solutions and problems with this approach:
1) Casters now lack an at-will attack that gets automatic damage scaling. Then again, martials don't get automatic scaling to their damage anymore either so it seems fair (aside from Rogue sneak attack which is niche). To compensate for magic weapon buffs casters could easily get magical implements of some kind, there's lots of precedent in fantasy roleplaying.
2) Casters can (at most) spend four spell slots per round (three for casting actions, one for their reaction). This means they can burn dry very quickly at lower levels, and have more of a reserve of power at higher levels. Since martials tend to get excellent passive abilities over time, this hopefully should compensate so the caster still has enough staying power to be relevant.
3) Making more spells Rituals means that magic is (1) more available to those who want to dabble in it and (2) less of an immediate problem-solving tool. For example, a spell that opens a locked door instantly might have a 15 or 30 minute ritual, which is occasionally useful if you lack a rogue and still need to get somewhere you shouldn't be. Yet it's still good utility to have in case you're being chased by a Troll and really need to get through that locked door RIGHT NOW. Utility rituals can easily be one Action to use normally so they drain fewer spell slots, leaving more power for the mage in combat while not being constant problem solving tools.
Ultimately I don't think this fix is going to happen, first because the rules are already firmly established, second because it's a bit of a departure from previous editions. Heck, if I had my way I'd divide spells into three categories like Feats, available at 1st, 8th, and then 14th level. But I know that such a thing would cause a riot in the community and tank Pathfinder 2.0 sales, so I'm sticking with the most reasonable suggestions in the hope that maybe some small part of this will resonate with the devs. Anyway, thanks for reading, feedback is appreciated as always.