Death 'sentance', doesn't have to be, depends where you're playing. As you get to higher levels/ages teams are likely going to get more sophisticated. I assume you are talking about one of your 3 rookie teams.
Basically what you are doing by only having one 2nd down is that you may not want the same plays called when you are 2nd and 1, and if you are 2nd and 15 or 2nd and 8.
The more detail you add the more refined you can make your offense. If in the 2nd down situation above you run an inside run that you typically get 2 yards on from 2nd and 8, you are going to end up needing a lot of yards on 3rd you might have been able to avoid if you used a play that might typically gain you more yards like a medium pass.
With that said, if you are going to consider adding more to your AI, don't try to add too much right away. You might want to break down 2nd down to a few different situations, a short, medium and long situation like you did for third down. Once you do that and are certain nothing is broken, you could expand your downs to where you are on the field. For example you may not want to throw long bombs on a 2nd and long in your own endzone. Or you might want to go for it on 4th down if you have a yard or less inside the opp 40 yard line.
