Reasonable:
1.) Change the outfield padding and replace with seating. There is a lot of wasted space between the outfield walls and the overhang seating. How about introducing extra seating there, somehow? Maybe include something similar to the first few rows of Petco's CF:
http://itsgametimesomewhere.com/wp-content/uploads/2010/07/PadresGameParkInThePark1.jpg
2.) Renovate the look and structure of the bullpen. Make it like the Yankees.
3.) Bring back fireworks. I don't know why they stopped doing that
4.) Include more areas to congregate and tailgate outside of the ballpark.
5.) Improve ventilation/cooling when the dome is closed during hot and muggy games