If you're going to get all the signs maybe down your back? Or 6 on each arm or 6 running down each leg

you want to make all signs? o.O
a friend of mine has Luhan's on her neck behind her ear

Ask a graphic shop to come up with a design
Maybe~

It will be cool if you can put it at the back of your neck, though you can only put one. But it's cool~

I thought of getting one as well, but well, only minseoks because he's my bias.. Don't you think all of them is a lot? Oo

I would choose a shoulder blade though ^^

If it wouldn't be too much for you, I would put in on your back and actually have the powers surrounding the tree of life. That would be cool or instead, place an EXO planet in the center and have the powers almost like its solar sytem, orbiting around it. Just my opinion :)

P.S I actually have pictures that would exactly show what I'm talking about if you would like for me to send them to you somehow :D