Last thread by Spartan13 had about a billion comments so....

Good + Realistic

Marcus Freeman: Not happening per Austin Ward

Jon Heacock Iowa State DC- Widely regarded as one of the best defensive minds in coaching circles. He helped create the new 3-3-5 defense that is spreading around college football that is designed to stop the spread while having defensive lineman who can still stop the run. From Ohio and coached Ohio high school football. Downside he is 61 and may not be a long term answer, also isn't known as a flashy recruiter

DJ Durkin Ole Miss DC- Not happening per Austin Ward

Kevin Steele free agent DC-Since 2007 Steele has been the DC for Bama, Clemson, LSU, and Auburn. At 63 he is very experienced in the south and is known as a bit of a mercenary who is brought in to fix defenses and make them go from bad to good and then move on. Steele could be a short term answer who can teach the younger coaches on the staff the right way to coach defense before Day finds a long term answer. Downside is this likely isn't a long term hire and may be a high floor/low ceiling move. Also Steele organized boosters to get Gus Malzahn fired last year in an attempt to secure the job for himself, maybe Day doesn't want to mess with that.

Mike Tressel UC DC-Consistantly solid at MSU and now UC. Was demoted at MSU after the 2019 season but is once again doing well at UC. Yes he is related to that Tressel. I wouldn't be a huge fan of this hire based on his tenure at MSU and lack of recruiting prowess but it is understandable why he would be a decent fallback option. He will likely have his defense playing sound at OSU but maybe not incredible. 
Morgan Scalley Utah DC-Former Utah player who has coached his entire career at Utah. They have consistently had one of the best defenses out west and at just 42 he already has 5 years of DC experience. Also typically gets the most out of a less talented roster so could be much better with OSU talent. However the Utah strong ties and allegations of racist remarks may make this unlikely.

Jim Leonhard Wisconsin DC-Regarded as one of the best DCs in the country former Wisconsin/NFL player would be a homerun hire. Consitently has one of the best defenses in the country at Wisconsin at just 39 and has plenty of experience. Being a Wisconsin alum would make this a really hard pull but Wisconsin pays him poorly at under $600k per year. He also has goals of being a HC and isn't getting the attention at Wisconsin that he would at OSU. He could triple his salary and be in a position to become a HC if he comes to OSU. 

Jim Knowles OkState DC-Likely going to win the Broyles award for best assistant after turning OkState into a defensive juggernaut. Former Ivy league player/coach is 56 and will be a hot commodity for bigger programs this offseason. Rumor is Florida is targeting him. 

Falling down the ladder(former head coaches who were fired)

TCU HC Gary Patternson-A legendary name in coaching known for his great defenses. However his old school approach and defense doing poorly led to his firing. Many have said they want him but I really don't see it. He is a bad recruiter and defenses just werent great. But maybe with a new fire and OSU level talent he could do something good.
Washington HC Jimmy Lake- After aggressively touching a player and being pretty bad on offense Lake was let go from Washington after just 13 games. He was a very good DC at Washington before hand and at just 44 it appears he will be in the market for a new job. A guy with experience at building a playoff caliber defense at Washington who is trying to get back up the coaching ladder I think would be a really good hire. With a massive buyout he may decide to take a year off but I think he would be solid. 
Charlie Strong/Chris Ash-Not fired but former Texas coach is currently an assistant for Urban in Jacksonville. Longtime DC with a ton of experience and known as a good DC could be a solid hire. We know Day listens to Urban and this could be a move Urban suggests thats in the good of Day and Strong. Ash similarly was fired from Rutgers and then Texas but he could come back to OSU. Has a ton of experience and could get OSU right. Neither are DCs and I think Ash could be realistic. 

NFL Guys: 

Matt Eberfluss: Colts DC mentioned on a Sirius XM radio show. Has significant college and recruiting experience as well. 

Ejiro Evero: My own personal favorite. Rams secondary coach who also coached with Ryan Day back when he was with the 49ers. Seems to be a rising star.

Derrick Ansley: Secondary coach for the Chargers, was previously DC at Tennessee and secondary coach at Alabama in 2017.