I think you are on the right track! Potentially, you should think a about a C part (you’ve got A and B), like a bridge that breaks up the “monotony” of A-B-A-B-… Many rock and pop songs use bridges - here, you could even consider getting away from the eighth note chugging and have longer sustained notes for a break (where, e.g., the drums could provide some fills). This’ll make it all the more interesting once you pick up the chugging again for the next A part.
First guess would be “B”, but I guess you are well aware of the discussion we had here: What key is this in? and how “important” (or not) the tonality is for a rock song… 