Necklines varies quite numerously but a few include collared, school uniform (I have no clue what they are called…), v-neck, and the scoop neck. For tank tops, most have the simple string but there are variations like the halter top which goes around or ties at the neck. For the average tank top, note that it rests just at the joint where the shoulder begins.
Sleeves varies just as much as the neckline! The top two are a close fit sleeve and a loose sleeve. Note the difference in fold lines and curves. The next is a cut sleeve which can be seen in modern clothes for women for a dressy look. The other, meanwhile, is more on the casual side for the prevalent baby-T’s, sporting an extra short sleeve.
Last, but not least, is the back of the top itself. Unlike the other areas of the top, the back remains fairly simple – a near exact version of the front except lines indicate the shoulder blades and the neckline is closer to the neck usually.
Tanks and other dressier tops, though, may have the scooped back exposing the shoulder blades and the spine.
As with any clothing, the best bet is to look through catalog’s and other references for ideas and variety.