Welcome!
When you talk to artists about the angel/cross idea, also ask them about future ideas for a full sleeve. I hate to say Google or Pinterest, but that could give you some ideas. Perhaps you should get the tattoo, and then think about it for a while before jumping in to add something else for the sake of filler or to complete a sleeve. You don't need to have the entire sleeve concept worked out to just get the part you want now. If something isn't crying out to be added, don't force it.