So I've been thinking about the Southern Magician and Precocious Apprentice feats.
Southern Magician is a regional feat from Races of Faerun. I allow people to ignore the region prerequisites of regional feats, though I might say that I wouldn't allow a character to take multiple regional feats with conflicting region prerequisites, on the assumption that many regional feats are probably balanced around a character being from only one region.
The relevant text of the feat is this: "Once per day per two spellcaster levels, you can cast a divine spell as an arcane spell, or vice versa."
"Psh," you might say, "That's a useless, crappy feat." And yes, yes it is.
Except for the mystic theurge's prerequisite: "Able to cast 2nd-level divine spells and 2nd-level arcane spells."
With Southern Magician, you can drastically reduce the level at which you qualify for Mystic Theurge. It still requires 6 ranks in two knowledge skills, so you'll never be able to reduce it below 4 (without Inspire Courage + Psychic Reformation shenanigans), but that's still better than the level 7 which would otherwise be the minimum.
Consider also Precocious Apprentice, from Complete Arcane. The relevant text: "Choose one 2nd-level spell from a school of magic you have access to. You gain an extra 2nd-level spell slot that must be used initially to cast only the chosen spell."
Precocious Apprentice allows you to cast a 2nd-level spell at 1st level. Southern Magician allows you to cast any divine spell as arcane and any arcane spell as divine. With these two feats, you meet the "Able to cast 2nd-level divine spells and 2nd-level arcane spells" requirement at level 1.
You'd obviously want to take a level in a second spellcasting class so you can actually derive some benefit from Mystic Theurge, and then you could Inspire Courage + Psychic Reformation your way into meeting the skill requirements, and take your first level of Mystic Theurge at 3, losing only 1 caster level in each of your casting classes.
This is all well-known trickery in optimizing circles.
Here's the important question: how much of this should a DM allow?
Mystic Theurge is one of those classes that a novice will look at and say "holy amazeballs, that's a ludicrously overpowered class!", and a more experienced player will look at and issue a resounding "meh". (Unless you involve Ur-Priest or Sublime Chord, in which case an experienced player will perk right up again.)
The problem is that, if you enter it as intended, even with wizard 3/archivist 3, the most efficient combination that even remotely resembles the intent of the designers, you're still 3 levels behind on both sides. You're breaking the First Rule of Practical Optimization: Thou Shalt Not Lose Caster Levels. Sure, you go to bed with more spells than a one-class caster starts the day with, but you'll be 1-2 spell levels behind for the rest of your career. Most optimizers agree the tradeoff is far from worth it, especially if your DM permits the 15-minute adventuring day (which he shouldn't).
If, on the other hand, you enter Mystic Theurge at level 3 (as a wizard/archivist), then you're only ever 1 caster level behind. Your spell level is on par with a sorcerer's, but you have more spells and the greater day-to-day flexibility of a prepared caster. That really is absurdly powerful.
Entry with Sorcerer 1/Favored Soul 1 is probably still tier 2, albeit extremely high tier 2; they get twice as many spells known and spells/day, but even that wouldn't bring their day-to-day flexibility up to the level of a prepared caster.
So, given that Sorcerer 1/Favored Soul 1/Mystic Theurge builds are probably at best on par with straight tier 1, it's a little incoherent to ban it without also banning all of tier 1.
But a Wizard 1/Archivist 1/Mystic Theurge build is approximately what you might call god tier, so not banning it would be insanity.
The obvious solution: make Southern Magician and Precocious Apprentice spontaneous-only. And, because casters really don't need nice things, let's make them mutually exclusive, just in case I'm wrong about the tier of Sorcerer 1/Favored Soul 1/Mystic Theurge, and thereby make it at least 4 of one, 1 of the other. Which is still 1 level earlier than prepared casters can enter; maybe this will make prepared casters not quite so much the obvious choice.
As in Complete Arcane, except:
Special: You can take this feat only as a 1st-level character.
You cannot take this feat if you have any ability to cast prepared spells. If you gain the ability to cast prepared spells, you lose the benefit of this feat.
You cannot take this feat if you have the Southern Magician feat.
As in Races of Faerun, except:
Special: You cannot take this feat if you have any ability to cast prepared spells. If you gain the ability to cast prepared spells, you lose the benefit of this feat.
You cannot take this feat if you have the Precocious Apprentice feat.