Being a branching woody bush plant rosemary is a suitable bonsai subject, don't know as how its a quick bonsai, seems to me it would need to have the branches wired down. I think putting it in the earth to thicken up would be a good idea, but if you want a very small plant you could just use what you have now, goodluck.

Planting in the ground isn't an option for me.

I guess just deciding what shape I'd like it to be and take out *everything that doesn't fit*? LOL

I did one as a quasi-topiary once (used it as a 8 inch Xmas tree) and that was fun.

Perhaps a rather largish pot for a few years then? I just get the feeling that you won't be terribly pleased with to look of it if its so skinney.