I was an English major in college, so maybe I'm taking some background for granted here: no one expected that good poetry had 5x14 da-DUMs without relief. (Certainly you wouldn't want to force Shakespeare to sound that way when you read it aloud.) But the most common variations were well-known and tended to feel right, letting the poet vary things enough to avoid sing-songiness but still not stand out as wrong. So a trochee in the first foot is very common. Same with a spondee in the last foot. Other feet can flip to become trochees too, but people have pointed out that it is more common in some feet than others. For example (IIRC but it's been a long time) I don't think the fourth foot flips very often, because it's hard to make it not sound bad. But those changes are "allowed" and so "it's still iambic pentameter." So it'd be cool if your algorithms could acknowledge those deviations, e.g. maybe give the line a purple-and-white shading instead of just white, or an asterisk.
EDIT: A quick Google turned up this article that has a lot of the patterns I remember: https://versemeter.wordpress.com/2016/09/20/iambic-pentamete...