So in Pathfinder 2e gaining some additional spellcasting options is pretty straightforward — pick up the Multiclass Dedication feat for any full spellcasting class other than your base class, and then start grabbing Spellcasting feats as they become available. Of course that gets you spells from another class, rather than your own, and it requires you pick up a Dedication feat first. But when it comes to gaining more spell slots for your own class, your only real option is Cantrip Expansion. But cantrip Expansion is very similar to the Spellcasting dedication feats, in that it gives you access to two extra cantrips.

So, let’s compare the Cantrip Expansion feat to the Wizard Dedication feat:

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CANTRIP EXPANSION
Feat 2
Bard Cleric Magus Oracle Psychic Sorcerer Witch Wizard
A greater understanding of your magic broadens your range of simple spells.
Prepared Caster (Cleric, Wizard, etc.): You can prepare two additional cantrips each day.
Spontaneous Caster (Bard, Sorcerer, etc.): Add two additional cantrips from your spell list to your repertoire.

WIZARD DEDICATION
Feat 2
Archetype Dedication Multiclass
Archetype Wizard
Prerequisites Intelligence 14
You cast spells like a wizard, gaining a spellbook with four common arcane cantrips of your choice. You gain the Cast a Spell activity. You can prepare two cantrips each day from your spellbook. You’re trained in arcane spell attack rolls and spell DCs. Your key spellcasting ability for wizard archetype spells is Int, and they are arcane wizard spells. You become trained in Arcana; if you were already trained in Arcana, you instead become trained in a skill of your choice. Select one arcane school of magic; you don’t gain any abilities from your choice of school.
Special You can’t select another dedication feat until you have gained two other feats from the wizard archetype.

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So we can see from this that the Wizard Dedication feat gives you much more than the cantrip Expansion feat — but both add 2 cantrips per day to your total spell output. Now, there are potentially good and valid reasons for the Dedication feat to give you more, the biggest one being that as a dedication feat once you take it, you are temporarily locked out of other dedication feats, while Cantrip Expansion leaves you free to still pick up an archetype if you wish to.

So, using this as the power level baseline, we can create other Expansion feats as class feats by comparing them to Sorcerer/Wizard archetype feats of the same level. That gives us a clear blueprint for Spell Slot Expansion class feats, which like Cantrip Expansion) become class feats for all the spellcasting classes.

(Art by warmtail)

Basic Spell Slot Expansion Feat 4
Bard Cleric Magus Oracle Psychic Sorcerer Witch Wizard
A greater understanding of your magic broadens your range of more complex spells.
You gain an additional 1st-level spell slot. You gain an additional 2nd-level spell slot when your class level reaches 6th, and an additional 3rd-level spell slot when your class level reaches 8th.

Spell Breadth Feat 8
Bard Cleric Magus Oracle Psychic Sorcerer Witch Wizard
Prerequisites Basic Spell Slot Expansion
A greater understanding of your magic broadens your range of more complex spells.
You can cast more spells each day. Increase the spell slots you gain from your class by 1 for each spell level other than your two highest class spell slots.

Expert Spell Slot Expansion Feat 12
Bard Cleric Magus Oracle Psychic Sorcerer Witch Wizard
Prerequisites Basic Spell Slot Expansion
A greater understanding of your magic broadens your range of even more complex spells.
You gain an additional 4th-level spell slot. You gain an additional 5th-level spell slot when your class level reaches 14th, and an additional 6th-level spell slot when your class level reaches 16th.

Master Spell Slot Expansion Feat 18
Bard Cleric Magus Oracle Psychic Sorcerer Witch Wizard
Prerequisites Basic Spell Slot Expansion, Expert Spell Slot Expansion
A greater understanding of your magic broadens your range of extremely complex spells.
You gain an additional 7th-level spell slot. You gain an additional 8th-level spell slot when your class level reaches 20th.

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