Hmmm I understand. Also if you design the t-shirt at size S or M and then you make the arms XXXXL it will be visually confusing. So if you want to stick with the alien theme, turn the t-shirt into a sleeveless one.
About art direction, the problem is not which way to go but that you are undecided. When you create art you need to have a specific vision before you begin. It might change during the actual creation process but you need a "blueprint", so here's my advice:
Decide what you want. Alien or human? After that make the appropriate research and stick to it. You might one day have to face a similar situation in a commission or a freelance job (or in life) so this is perfect practice! You will gain some hardcore work-ethic experience points from this!
You might ask "but how may I decide?" That's easy! All you have to do is to think what kind of feeling you want to evoke to the viewer. Is she scary? violent? mysterious? sexy? ugly? casual? Then after you decide, think what characteristics can go with that. for example she has on her left arm a wristband. If she is violent put spikes on it. If she is nerdy make it a flashy touchscreen. If she is beatiful put emphasis on her lips etc.
After you decide, create a low resolution image and make thumbnails like this:
The low res would be so you don't lose time drawing details like nostrils No color, just greys and try to make as many variations as possible. You will find the one image that feels right and then my friend you just render on a higher scale image Trust your instincts, and if you need any other help I will do my best to help you.