If Allardyce does leave after relegation (not a guarantee considering this club loves to reward failure) then Marco Silva would be top of my list.
I would like us to take another look at Bruno Labbadia though. He was interviewed for the job a few weeks before Bilic was appointed. He is known in Germany as the guy a club brings in to save them from relegation but he hates that reputation and wants to be able to build something at a club. That didn't work with Hertha Berlin this season but that club has been a mess for a couple of years now.
I would like us to take another look at Bruno Labbadia though. He was interviewed for the job a few weeks before Bilic was appointed. He is known in Germany as the guy a club brings in to save them from relegation but he hates that reputation and wants to be able to build something at a club. That didn't work with Hertha Berlin this season but that club has been a mess for a couple of years now.