What format should my work be?

I think you won't know until you start creating it. You have a well formed idea, but as far a the reception of that idea goes: there is really no way of knowing until the product is done and marketable.

Focus on the story and/or art, because in the end, the quality of the work is the most important aspect. Your format may change as you work on it and you may find new ways of presenting and combining art and writing. Don't pigeonhole yourself before you've even started. Enough people will do that for you when you are done.

If you haven't written a graphic novel script before, you could try to find a writing partner or a mentor. There are Graphic Novels that are drawn/written by a team.

Just get the story out of your head in whatever way it develops organically and then work forward from there.

Best of luck!