You made me hungry.
Bell pepper adds a distinctive flavor to the beans, and is part of the "trinity" used by Cajun and Creole cooks. A hotter pepper wouldn't hurt either. I usually use Anaheim peppers and jalapeno peppers in my red beans and rice.
I see many have already got here before me with good answers . Don't however leave out pepper , it's red beans and rice .
A poblano will be fine , any nice green pepper will do . I prefer the hot ones myself . I just remove the seeds so it only has a spicy bite and does not just burn everyone . LOL Even my son will eat it then .