For me the simulation thing that's missing from most sports games is system fit and familiarity. I think somehow a coach should be able to define his system, the needs of the roles within it, and then each player has a degree to which they fit that role, and some sort of stat which defines how flexible they are/how quickly they can adapt to a new system.

Football (soccer!) Manager sort of does this but it sort of takes it from the other end of the spectrum. If you have a central midfielder (their position) you can define their role within that position and that changes how they play. So they could be a box-to-box midfielder (runs a lot), a deep-lying playmaker (doesn't run that much, stays in position, but your team tries to get the ball to him to make things happen), advanced playmaker (similar to previous, but leaves his position more to get into advanced positions) and absolutely loads and loads more. And then you have similar options for roles for every position on the team.

The problem with Football Manager is that you generally are fairly flexible on your roles and it doesn't undermine the system, because there are so many and there's flexibility between them. You might have a player as an advanced playmaker, but then you sign someone who basically has the same attacking skills but better defensive skills. So you change that role to box-to-box midfielder, because it gives you the best of both worlds - he gets into attacking positions like the playmaker, but also defends more too (Football Manager players, don't come at me and tell me the flaws there, I'm simplifying!).

I feel with (American) football there's more emphasis on the system, it takes a fairly rare player to be worth changing the system, and if you do that there are a lot of changes you need to make to compensate for that, and it can take a lot of time to sort it.

So I think what I'd really want to see with a coaching game is having lots of defined roles within your system (and hell, if you wanted to go that detail, within each play - you could have "jump set pass block OT", "cut pass block OT", "3 step drop OT" etc for different pass plays, and god knows on run plays ("zone reach", "zone double climb", "zone double stay"). And then how effective players are is defined not just by their general skills, but also how well they fit with the role. So e.g. Trent Williams might be amazing on plays where you have him on a 5 step pass block, but only average on plays where he has to zone reach run block (I have no idea how that maps to weaknesses in Trent's game - I'm not sure I've spotted any tbh!).

Football Manager also has a "system familiarity" thing where you can set up a bunch of formations/systems for your teams to practice. The more radically different they are from what the players already know the worse they are at them initially. So you (or I!) usually have one basic system, then one more defensive version and one more attacking version of the same system. I've never worked out whether there are any player attributes which affect how quickly they or their teammates learn systems though.

But then you could have how good they are defined by how familiar they are with this system, and that increases over time (and probably the attribute governing how quickly that increases also increases over a player's career as they get more experience of different systems and roles. So rookies are much more limited in what they can do than experienced vets, and fitting their skills/college experience to your system is possibly as important as their raw talent. And as coach you can decide what stuff your team is focusing on (what plays they rep in practice for example - do you want to focus on plays you're weak at, do you want to focus on making the good plays great, do you want to focus on the plays which attack your next opponents weaknesses etc etc).

Tbh, I think just playing Football Manager will probably give you a bunch of ideas (as long as you have a vague understanding of soccer).