It took a bit but I finally organized my thoughts.
South Endzone
I would like to see something similar to what
@Fighter of the Nightman sketched up. A rebuilt horseshoe that is pulled closer to the back of the EZ and either two upper deck wings with a scoreboard in the middle - similar to Iowa State - or a complete upper deck with corner screens - like LSU. The lower hall would be updated with various amenities, the upper hall(s) would also offer many good options. Pulling things in toward the back of the endzone, Kirby should not be affected but the outer wall may be right against the sidewalk.
The SEZ currently seats ~10k. Assuming that the new structure closes in the bottom ~15 rows for ~6 sections - with a tunnel in the middle - and moves a bit closer to the back of the endzone, let's call that would adding ~1k seats. For the upper deck, assuming 25 rows x 10 sections x 30 seats wide of standard seats and ~1k premium seats, that will add another ~8.5k. Total add would be +9.5k.
North Endzone
I would like to see something like Fighter's modified version of the ~2016 structure - wall of fans, beer gardens in the lower corners, recruiting lounge high up, good-sized scoreboard behind/above, etc. Having the Block I and the MI sitting immediately behind the EZ would be intimidating for opposing teams. My only adjustment would be to have a tunnel in the middle. That way, during pregame, the MI can split the big Block I formation in half and then the team and cheerleaders can take the field down the middle. The opposing team portal/tunnel will still be in one of the corners. The hall in the NEZ would be spacious, with plenty of amenities.
I estimate the currently NEZ seats ~4.5k. The wall will likely seat ~12k (60 rows x 25 people x 8 sections), for a delta of 9.5k.
East Sideline
Remove ~25 rows from the lower deck and replace them with premium seats to mirror the WSL. As discussed in an earlier thread, the ESL hall can then be updated to mirror the WSL - new restrooms, food vendors, etc.
Removing the upper 25 rows would take out ~6.5k seats. There would then be ~1.2k premium seats added, for a delta of -5.4k.
Summary
This revised stadium would pull-in the SEZ and provide an upper deck for additional fans, have a 'wall' in the NEZ, and would open up the ESL Hall. The SEZ and NEZ structures will close the gaps between the ESL and WSL towers, helping keep sound in. The video boards in the NEZ and SEZ will allow fans to see replays and announcements without having to turn around.
The new seating capacity would now be ~74.2k (60.6k + 9.5k + 9.5k - 5.4k) and that feels like a good number for Illinois to shoot for. That would put us at 8th in capacity in the B1G - between MSU and Iowa - and around 30th overall for the US.
What are your thoughts? Special thanks again to
@Fighter of the Nightman for most of the visuals that I referenced.