I would've prefered the main Humans (Dallas, Ricky, Kelly, etc.) being the cause of the cities destruction. Instead of simply trying to escape, the main characters would've seen how much damage the Aliens could do. Consequentaly, while most humans are being captured or killed, they would've sabotaged the Nuclear Power Plant. They would arrive at the hospital, then fly to the plant. Dallas would stay behind to make sure the plant explodes and hold back the aliens while Ricky, Kelly and Molly escape.
Multiple Military Helicopters would've arrived to pick up any surviving civilians. Some copters would be taken out by aliens, and the rest would be destroyed by the explosion.
During this time, Wolf and Chet are fighting. The fight would've been longer, and the build up to the explosion would've been greater.
These are only some of the changes I would have made. Whilst watching the movie, I came up with a lot of changes in my head.