Just type during the game:

 Game improvement:

 gloves        Better catching
 bigfoot       Better kicking
 leech         Better defensive back

 New stadiums:

 ea stadiums   EA Stadium
 dogpound99    Cleveland Stadium
 notafish      Old Miami Stadium
 thehogs       RFK Stadium
 sombrero      Old Tampa Bay Stadium
 for rent      Astrodome
 ourhouse      Tiburon Stadium
 stickem       Original Oakland Stadium

 New teams:

 welcomeback   '99 Cleveland Browns
 bestnfc       NFC All Stars
 afcbest       AFC All Stars
 boom          Madden '98
 imtheman      Stats Leaders
 peacelove     All 60's
 bellbottoms   All 70's
 hereandnow    All 90's
 turkeyleg     Madden All Time Greats
 throwback     75th Anniversary
 gearguys      NFL Equipment Team
 orrs heroes   EA Sports Team
 jetsons       Old Oilers
 ghost town    WildWest
 dandaman      Old Miami Dolphins
 sharksfin     Tiburon Sports Complex