I know lately more teams are playing a 4-2-5 coverage instead of a traditional 4-3.
sure you can say thats just nickel coverage, but in this case, the 5th DB is more of a hybrid LB/safety that can do everything. Fast enough to cover a WR when the team is passing, tough enough to make a tackle when the other team is running.
Of course you could extend that to ridiculous lengths by essentially playing a prevent defense with 4 DL and 7 DB's.