Alex Van Pelt
Alex Van Pelt was with the Buffalo Bills for close to 10 years as a player from 1994 to 2003 primarily as a backup. After retiring, he has gone on to put together an extensive resume as a coach where he has been quarterback coach for four NFL teams (Bills, Bucs, Packers, Bengals, and Browns) while also being offensive coordinator for the Bills and recently the Browns.
The Browns have never been considered one of those top offenses with Alex Van Pelt calling the plays but one area they have excelled is on the ground. However, late in the year with Joe Flacco under center the passing game did open up more and there is no reason the Bills couldn't have something similar with Van Pelt calling plays.
Continuing with the Buffalo Bills connection, former backup quarterback Frank Reich could be another option. He certainly has a wealth of experience as he had two years as offensive coordinator with the Chargers in 2014 and 2015, two years with the Eagles in 2016 and 2017 (which included a Super Bowl ring in 2017).
Reich would go on to be a head coach for five seasons with the Colts and this past season was head coach of the Panthers but was fired midseason. While he wasn't able to turn the Panthers offense around this past year, he does have a long track record being successful on the offensive side of the ball and this is something that could benefit Josh Allen.