Personally, I would like to see some random "Drizzit" (did I spell it right?) rules, i.e. any race/class combination. Some possibilities are:
Dark Elf Ranger
High Elf Shadow Knight
Erudite Warrior
Ogre Cleric
Barbarian Paladin
Iksar Rogue
Human Shaman
Halfling Necromancer
Gnome Bard
Troll Bard (hehe)
You would maintain the same starting stats for that race. For example, the High Elf Shadow Knight would have the same stats for a Paladin with the same additional points to distribute. The Orge Cleric would have the same base stats as the Orge Shaman plus the additional points.
The character would be shunned by its original race, perhaps even outright KOS. Their faction would be capped at dubious at best. On the flip side, the character would be dubious with their new class in other cities, with the possibility of raising their faction. So a Dark Elf Ranger would be Dubious to the wood elven rangers. I agree that some thought would have to go into the coding of the factions, but no different than any other new race.
A disadvantage is that you would have to choose the appropriate deity. This would cause you to miss out on some cultural/deity armor and items. For example, gnomish plate armor is tagged for warriors, clerics, paladins and shadow knights; therefore, a gnome bard would be out of luck. I think some of the older armor (Kunark Class armor) might also be restricted.
I would also like to see this as a one time activation for each account. So you would get one shot at making a unique race/class character. If you get to level 10 and realize your selection sucks, then you are out of luck and you do not get another choice.
The combinations would be more fun for the races that have limited selections, like barbarians and ogres. An Ogre Mage (in a robe) would be hilarious. I would much prefer this one time oppurtunity instead of a new race.