The biggest theoretical draw IMO is presenting something uniquely racial. Not in the sense that elves can only be Knights of Frilly Things but in the sense that Knights of Frilly Things can only be elves. I realize that's not how racial classes have been in the past, but I think that ideally, this is how I'd like to see racial classes presented in the future. Not simply hybrids of existing classes or weapon-focus classes like "Dwarf Mountain Crusher" is really just "Fighter who focuses on hammers and wears rocks", but along the lines of the way in which 4e Essentials classes were basically pre-built to give the player a functional character around a very specific theme.
Racial classes are the niche of the niche. You're not just an elf who wears shiny armor and lacy underwear, you're an elf who focuses specifically on combat styles that revolve around fighting in and fighting with shiny lacy underwear. They offer little room for variance because they are designed to create a very specific concept and theme for that race.
At least that's how I feel about racial classes. I'd be fine with every race having a "racial class" that exemplifies a very very VERY specific theme or concept of that race. I'm not sure I'd agree with every conception, but as long as they attempted to avoid tropes like making Dwarves "Hairy Drunken Hammer Fighters" I think we'd see some very creative stuff.