The "owner mode" stuff from years ago was just about the dumbest aspect of Madden ever. Why the hell do I care about concession stand prices or stadium upgrades, if I can't USE those features when I play the game.
To my mind, the single biggest thing they could do to make this game better would be to upgrade the sounds of the game. It should sound like really being there, down to the minute details.
I want to hear stadium specific EVERYTHING. The atmosphere of a real NFL game is incredible, and the game isn't even close to recreating that. Instead, they try to recreate the sound of a television broadcast (which even in real life is pretty dull) and the game sounds worse (much worse, actually) than television.
The second thing they could do is make the TIMING of the game more like a real NFL game. Now, nobody wants to sit through 3 hours of a single game. But lets face it... MLB The Show has nine full innings, with three outs per inning, and we still play that! No more "five minute quarters." where you play a full game, and your offense only runs 30 plays, but your QB has 3 TD passes, and your RB is over 100 yards. I'm sorry, that's silly. The game has been tweaked for years to make it so final game stats look realistic, and season stats are realistic, but the average yards per play has to be WAY over the top, in order to get to those numbers on 1/3rd the amount of play time. All games in Madden should have 15 minute quarters, with an accelerated clock feature. THEN, adjust the difficulty so that the results of each play are realistic. Not just the overall stats.