The ability to family the hairs, so if you switch from color to color, your Sim keeps the same hair.
Another awesome thing I'd want to see, though I have no idea if it would actually be feasible, is the ability to remove the shoes off of an outfit. Like swimwear with high heels, blech. Maybe Seasons would actually make that part easier, I dunno.
I know this is quite an old topic but I just thought I would stop by and mention that I used to use DatGen to family my hairs. I never quite worked out how to do anything else with it but it worked really well for that task!