'The Book of Souls' was successful because the songs were good, not because they were long. It's possible to overreach, to be ambitious and fail. It's also possible to be ambitious and not produce bloated, overblown material.
For example, 'Hallowed be thy Name' is the perfect length and would suck if it was as long as some of the songs on the last two albums. 'The Red and the Black', however, would be a much better and more digestible song if it were only 6 or 7 minutes long.
I'd like Iron Maiden to make good songs for the sake of making good songs, not long songs for the sake of making long songs.
Ultimately they'll make whatever they are inspired to make, be that short songs or long songs.