Esker
Hero
For AT and EK specifically, I've kicked around a variant multiclassing option along the lines of the following: when you level up, you can choose to devote a level to your casting ability. When doing so, you do not receive any new features from your class or subclass, but your caster level is increased by 1, which bumps up both your spell slots and your spells known as if you had attained 3 levels in your class.
So it would basically be like taking a wizard level, but you don't start up a second list of spells known and you don't get wizard class features; instead you accelerate your caster progression from your main class.
So, for example, an arcane trickster could take a casting level at 6th, delaying expertise but learning two 2nd level spells as if they were 8th level in AT, one of which must be enchantment or illusion.
If they took two more caster levels, they would get another cantrip, 3rd level slots, two more 2nd level enchantment/illusion spells known and two 3rd level spells known, one of which must be enchantment/illusion.
They are now 8th level but have a caster level of 5, with a number of cantrips, wizard spells and spell slots as a 5th level wizard (but with the AT school restrictions). But not arcane recovery or ritual casting, nor a wizard subclass.
Then, if they went back to rogue, they'd get additional expertise at 9th, followed by evasion and a caster level bump at 10th (per 1/3 of 7 rounded up), giving one more 3rd level enchantment/illusion spell known as at AT 16.
So at 10th, you'd have 7th level rogue features and a limited version of 5th level wizard features: 11 spells known of 1st-3rd level, 4 wizard cantrips, and 4/3/3 spell slots.
Compare to an AT 7 / Wizard 3 under RAW, who has 11 1st level spells known, 4 2nd level spells, 6 cantrips, and 4/3/2 spell slots, ritual casting, two spell levels of arcane recovery, and a level 2 wizard subclass feature.
So it would basically be like taking a wizard level, but you don't start up a second list of spells known and you don't get wizard class features; instead you accelerate your caster progression from your main class.
So, for example, an arcane trickster could take a casting level at 6th, delaying expertise but learning two 2nd level spells as if they were 8th level in AT, one of which must be enchantment or illusion.
If they took two more caster levels, they would get another cantrip, 3rd level slots, two more 2nd level enchantment/illusion spells known and two 3rd level spells known, one of which must be enchantment/illusion.
They are now 8th level but have a caster level of 5, with a number of cantrips, wizard spells and spell slots as a 5th level wizard (but with the AT school restrictions). But not arcane recovery or ritual casting, nor a wizard subclass.
Then, if they went back to rogue, they'd get additional expertise at 9th, followed by evasion and a caster level bump at 10th (per 1/3 of 7 rounded up), giving one more 3rd level enchantment/illusion spell known as at AT 16.
So at 10th, you'd have 7th level rogue features and a limited version of 5th level wizard features: 11 spells known of 1st-3rd level, 4 wizard cantrips, and 4/3/3 spell slots.
Compare to an AT 7 / Wizard 3 under RAW, who has 11 1st level spells known, 4 2nd level spells, 6 cantrips, and 4/3/2 spell slots, ritual casting, two spell levels of arcane recovery, and a level 2 wizard subclass feature.