As for subject matter…You’d have to figure that out for yourself.
What are you into?
What inspires you?
Is there simply a style of art that is especially beautiful to you?
Right off of the top of my head…I can think of a tiger or possibly a rugged mountain terrain…water that flows with the marks…

Also, stretchmarks can be hard to hide with a tattoo.
From a few pictures that I’ve seen…they tend to ruin a beautiful piece more than the beautiful piece fixes the skin. Frankly…stretch marks aren’t a big deal anyway. I’m sure that you personally are far more self-concious about them than you need to be.
ALSO, if the stretchmarks are three dimensional…have valleys…they could prove problematic for the tattoo artist to even do a decent job on.

I don’t know if this will help you…
But I’m pretty sure shea butter is supposed to be good for stretch marks?
Maybe that is during the actual “stretching” phase though…

I wish you luck!

Depending on how the marks are laid out, you could use them as a part of the tattoo instead of just hiding them. If they are more spread out instead of a lot of lines right on top of each other you could create a prison within yourself tattoo. It could depict yourself as some sort of demon like thing (not like hey thats you hiding in there) being trapped, or beginning to escape from a cage/ jail, yourself on your stomach.

This could show how you escaped the person you once were, 100 pounds heavier, and the scars could be the positive area of the flesh bars of your prison, or the negative ares in between the bars. If that doesn’t make any sense its like a cave with the stalagmites and stalactites, and how they can form a wall of bars. That way you could include as much as you want, arm or leg ripping out, or and entire scene where it shows your inner demon breaking through some flesh bars.

just an idea