As long as we are spit-ballin pie in the sky ideas how about an open air stadium with the seating platforms heated as with in-floor systems. Keep the concrete mass warm throughout the FB season. When stadium not in use cover the seats with roll down insulation blanket that helps to retain the heat in the concrete mass rather than let it escape. Then on game days retract the blankets section by section just like baseball field covers.
Further, any partial roof over the tops of the stands could be completely covered with passive solar arrays. Sell excess output back to the utility. For main mass heating use ground source heat pumps with grids buried underneath all the parking areas.
Reminder - I'm just spit-ballin here.