IAD feels like it's been pretty stagnant recently (relative to the crazy movement happening in a lot of places right now, rather than historical averages). Not a lot of slots there in any fleet/seat on recent vacancies. This is just one data point, but at my particular seniority level, it's actually the 2nd most senior of the 11 bases for 737 FOs (with only Guam being more senior).

My crystal ball says that DC may eventually see significant growth 5-10 years down the road, especially if they can get the new terminals figured out. Not sure about anything before that though.