US or EU?
If US - Alliance is pretty low on numbers at the moment, and as for classes, they have quite a few melee DPS so I wouldn't suggest rolling one.
If you're interested in DPS, perhaps roll a boomkin, I haven't seen an Alliance boomkin in a while, or a spriest.
If interested in healing and want to play a low-pop class/spec - try resto shaman, or holy priest.