I agree with the Sorcerous Mage idea. As another stated, a Sor 10 / Wiz 10 is nothing compared with a straight Wiz 20 or Sor 20.
Hmm, I can see being a Sor 3 / Wiz 17, though. Basically it would be a Wiz 17 with a few spell-like abilities (up to 6 castings from a list of 5 cantrip spell-like abilities, and up to 5 castings of 3 1st level spell-like abilities). That would be good flavor if 1) the sorcerer levels were spread out (1 every five levels or so, so the PC doesn't fall to far behind the party), and 2) you do not allow a Sorcerous Mage PrC. The PrC idea is innately better, of course, but I can see how some DMs might not like the idea.
A theme for the sorcerer is nice, but not necessarily necessary. I usually give them a minor bloodline instead of a familiar. (Thus more differences between the Sor and Wiz, and it also helps explain the sources of the innate magic). I also give Sor the skill UMD, but that has long been a house rule. Hmm, if a theme is used, base it on the bloodline (fey bloodline: enchantments and illusions, etc; dragon bloodline: all based around a particular energy (ie: breath weapon of ancestor; and so on).
Actually, the more I think about it the more I like the idea of a Wiz 17 / Sor 3 (for an NPC). Imagine, always having low level defense / offence; arrested and spellbook taken? No problem! (assuming low level guards, etc) Of course, it would work better if the Wiz took Spell Mastery a time or two (for higher level spells).
But give real consideration to the Sorcerous Mage. I don't know how well it will balance, but as the Mystic Theurge barely breaks even it shouldn't be too over-powering. Do not, under any circumstances, allow the PC to be a SMg* if the player has shown signs of munchkinism in the past!
* SMg = Sorcerous Mage. It does not = Super Mage! Despite its potential for abuse at low to mid levels, it really starts to fall behind at higher levels. Remember that Sor receive spells a level late. A Sor 5 / Wiz 5 / SMg 10 can cast as a Wiz 15 / Sor 15. That means no 8th level Sor spells. Although I'll admit that the sheer number of low and mid level spells likely makes up for having only 1 8th level spell and no 9th level spells. Remember, even after 20 levels the class only has 4 7th level spells - as many as a Wiz 19 and only 1 more (known) than a Sor 20. Thus, it barely breaks even for 7th level spells in its composite classes - 4 levels later.
Get back to us with the final results, please. What ideas will work for you? (minor bloodline w/ or w/o a familiar; SMg PrC; ...)