This is a great game and very immersive. I have a couple of thoughts that may not affect game mechanics much, but might make it even more immersive
1 - either players have last names, or users can change player names. I kind of like giving them last names
2 - a player stats page so we can see how our favorite RB stacks up across the league
3 - a logo generator to customize the team look a bit
4 - adding to the players "years in the leageu" versus rookie and then maybe showing which teams they have been with on their stats page

A couple of other thoughts
5 - this is a bit more complicated, but it would be cool, as an owner to recruit a coach, oc, dc, special teams coach. Maybe they give bonuses to players.
6 - player awards (MVP, etc)

1+

1+

One more thought maybe..
I haven't had any injuries yet, so I'm not sure how they work, but it would be awesome if injured players could play at a negative, or have to sit, based on the injury and a backup has to be used. We could even get a brief description of the injury type.