Monk Multiclassing
If your group actually enforces the Monk multiclassing restriction, and you want to go back to Monk for some reason, Monastic Training from Eberron Campaign Setting allows you to multiclass Monk with any one other class. The only good use of this feat is Tashalatora (see Psionics below), since it’s very rare for anyone to leave Monk and then want to get back into it. Many players consider two levels of Monk to be the absolute maximum anyway.
Arcane Spellcasting
There are no feats that allow arcane spellcasting levels, of any type, to stack with Monk levels for the purposes of Unarmed Strike Damage, which is what you really want. Mixing arcane magic and monk features usually only happens by using Prestige Classes, such as the Enlightened Fist (Complete Arcane).
Various gish-related feats work for Monks as well as anyone else. Arcane Strike (Complete Warrior) is pretty good, Smiting Spell (Player’s Handbook II) is OK. Probably off-topic to go into a full gish guide here. Note that Enlightened Fist is much better at doing what Smiting Spell does than Smiting Spell itself is.
Intelligence-based Casting
You can take Carmendine Monk (Knights of Valor) or Kung-fu Genius (Dragon vol. 319) to change the Wisdom to AC bonus to Intelligence to AC. Carmendine Monk also counts your Monk level as 2 higher for the purposes of one of the following (chosen daily): unarmed strike damage, unarmored AC bonus, or fast movement. Neither’s particularly good, but they exist.
By the way, the Carmendine Monk feat doesn’t actually require Wizard at all; you can just use it to make an Int-based Monk. For a Monk/Wizard, though, it’s what you've got, since it’s strictly better than Kung-fu Genius.
Charisma-based Casting
Ascetic Mage switches Wisdom to AC to Charisma to AC, and lets you stack Sorcerer and Monk levels for the AC bonus. It also lets you trade spell slots for an attack/damage bonus to unarmed strikes equal to the spell level, as a swift action.
This is not a particularly good feat, since the stacking only applies to the AC bonus (and that grows extremely slowly). The effect for sacrificing a spell slot is decidedly inferior to Arcane Strike (Complete Warrior).
Invocations
There are no feats or prestige classes that directly stack Invocations with Monk class features, and you cannot qualify for the ordinary options above due to the way Warlocks and Dragonfire Adepts qualify for feats and prestige classes.
Precocious Apprentice (Complete Arcane) can arguably get you a 2nd-level spell as a Dragonfire Adept or Warlock. Good luck finding a table that will allow it. If you do, you can then take Enlightened Fist and/or Ascetic Mage.
The Eldritch Claws feat (Dragon vol. 358) gives a Warlock a pair of Claws that deal damage equal to his eldritch blast damage plus his unarmed strike damage. Obviously, this is better if your unarmed strike damage is good. The Beast Strike feat (Dragon vol. 355) allows your unarmed strikes to deal damage equal to your normal unarmed strike damage plus your Claw (or Slam) damage. Combine the two to deal unarmed strike damage + unarmed strike damage + eldritch blast damage as an unarmed strike attack. I.e. you can Flurry it. This combination is not particularly appreciated by most tables, however.
The Shou Disciple prestige class is much better than Monk for improving your unarmed strike damage for this purpose, though.
Psionics
You did not mention Psionics, but I want to point them out because they have a much better option for Monks than do any other sort of magic.
Tashalatora from Secrets of Sarlona allows you to choose any one Psionic class as progressing your Monk class features, and can be taken as a Monk Bonus Feat at Monk 2. It requires Monastic Training (Eberron Campaign Setting) in the desired Psionic class, which itself can be taken as a Monk Bonus Feat at Monk 1.
Thus, a Monk 2/Psychic Warrior 18 or Monk 2/Ardent 18 (Ardent is from Complete Psionics) becomes a very solid build for an unarmed mystic combatant (both classes are ¾ BAB and depend on Wisdom). Practiced Manifester (Complete Psionics) is a must at 3rd level, especially for Ardents, though.
However, strictly speaking, neither Monastic Training nor Tashalatora explicitly requires Monk levels. Which means you can be a Human Psychic Warrior or Human Ardent who has full unarmed strike progression, flurry of blows progression, and AC bonus progression of a Monk of the same level at every level of the game. This is even better, since it protects your Manifesting, which is way better than what Monk 2 offers, even if it does cost you a couple of feats (one of you basically “get back” since you don’t need Practiced Manifester). Of course, many tables aren’t going to allow that.
