Well the Westfalenstadion is only 67,000 seats (80,000 with terracing and seats) , Started off with 54,000 but was later reduced to 42,800 (26k seats) but then went back up to 54,000(38,500 seats) then it went to 68,000 then the seating capacity went up from 52,000 to 67,000 seats for the world cup
Overall it had 3 phases, it started off with just 4 stands no corners, then 3 stands got 2nd tiers and the single biggest stand in europe (die Südtribüne) then the 3rd phase was adding the corners i think!
Not saying we need to go down the same route but we all know what a great stadium the Westfalenstadion is and its something we could get inspiration from!