I actually enjoyed the invasion angle. Although I will be the first to admit it could have been so much better. I also didn't like how it ended with WCWs top stars being Austin and Angle, 2 WWF guys.
I think the best bet would have been to have had Shane merge with ECW and have them take over smackdowns time slot. WWF and WCW should not even acknowledge each other at this point. In February bring in the NWO to "invade" WCW with bischoff and have them join WCW and challenge WWF.
Scott hall would come through the crowd and interrupt a WWF match just like he did to start the WCW/NWO angle.
Now you have Hogan, Hall, Nash, Flair, Booker T, DDP, and top the ECW guys to invade WWF.
Wrestlemania 18 would be WWF vs WCW. Possibly could get Sting and Goldberg for the event.